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faculty, and, in preparing a book by which this work may be 
commenced, she has met the profpundest need of popular edu- 
cation. 

D. APPLETON & CO., Publishers, 

New York. 



LOCKYER'S ASTRONOMY. 

ELEMENTS OF ASTRONOMY: 

Accompanied with numerous Illustrations, a Colored Repre- , 
sentation of the Solar, Stellar, and Nebular Spectra, 
and Celestial Charts ot the Northern 
and the Southern Hemisphere. 

By J. Norman Lockyer. 

American edition, revised and specially adapted to the Schools 

of the United States. 

i-zmo. '^\'2 pages. Price, '^i.$o. 

The volume is as practical as possible. To aid the student 
m identifying the stars and constellations, the fine Celestial 
Charts of Arago, which answer all the purposes of a costly Atlas 
of the Heavens, are appended to the work — this being the only 
text-book, as far as the Publishers are aware, that possesses this 
great advantage. Directions are given for finding the most in- 
teresting objects in the heavens at certain hours on different 
evenings throughout the year. Every device is used to make 
the study interesting; and the Publishers feel assured that 
teachers who once try this book will be unwilling to exchange 
it for any other. 

D. APPLETON & CO., Publishers, 

New York. 



Digitized by the Internet Arcinive 
in 2011 witin funding from 
The Library of Congress 



http://www.archive.org/details/physicalgeographOOgeik 



SCIENCE PRIMERS, edited hy • 

Professors ■ HuxLEY, ROSCOE, and 

Balfour Stewart. 



IV. 

PHYSICAL GEOGRAPHY, 



Smna IBnmtrs. 



d 



PHYSICAL 



GEOGRAPHY. 



BY 



^^ ARCHIBALD GEIKIE, LL.D., F.R.S., 

Director of the Geological Survey of Scotland, and Murchison-Pro/essor 
of Geology and Mineralogy in the University of Edinburgh. 



WITH ILLUSTRATIONS. 



NEW YORK: 

D. APPLETON AND COMPANY, 

I, 3, AND 5 BOND STREET. 

I 880. 










6q 



CONTENTS. 



ART. PAGE 

I 



^3 



Introduction i i6 

The Shape of the Earth . . . . , 17 — 26 

Day and Night 27—38 

The Air :— 

I. What the Air is made of .... 39—44 16 
II, The Warming and Cooling of the 

Air 45-60 

III. What happens when Air is warmed 

or cooled — Wind 61 69 

IV. The Vapour in the Air — Evapora- 

tion and Condensation . . . 70 81 

V. Dew, Mist, Clouds 82—89 "i 

vr. Where Rain and Snow come from . 90 — 97 35 

Summary 98 38 

The Circulation of Water on the 
Land : — 

I. What becomes of the Rain . . . 99 — 107 39 

n. How Springs are formed .... 108 — 116 42 

III. The Work of Water underground . 117— 125 47 



19 

24 
27 



vi CONTENTS. 



ART. PAGE 

IV. How the Surface of the Earth 

crumbles away 126 — 142 51 

V. What becomes of the crumbled parts 

of Rocks. How Soil is made , 143 — 153 cS 

VI. Brooks and Rivers. Their Origin . 154 — 168 62 

Summary 169 67 

VII. Brooks and Rivers. Their Work . 170—182 68 

VIII. Snow-fields and Glaciers .... 183 — 203 75 

The Sea : — 

I. Grouping of Sea and Land . . . 204 — 211 86 

II. Why the Sea is Salt ..... 212—216 88 

III. The Motions of the Sea .... 217 — 232 90 

IV. The Bottom of the Sea .... 233 — 251 95 

The Inside of THii Earth 252 — 265 102 

Conclusion 266- -268 109 

Questions iii 



SCIENCE PRIMERS. 

PHYSICAL GEOGRAPHY. 



INTRODUCTION. 

I. Let us suppose that it is summer-time, that you 
are in the country, and that you have fixed upon a 
certain day for a hoUday ramble. Some of you are 
going to gather wildflowers, some to collect pebbles, 
and some without any very definite aim beyond the 
love of the hoHday and of any sport or adventure 
which it may bring with it. Soon alter sunrise on the 
eventful day you are awake, and great is your delight 
to find the sky clear and the sun shining warmly. It 
is arranged, however, that you do not start until after 
breakfast-time, and meanwhile you busy yourselves in 
getting ready all the baskets and sticks and other gear 
of which you are to make use during the day. But the 
brightness of the morning begins to get dimmed. The 
few clouds which were to be seen at first have grown 
large, and seem evidently gathering together for a storm. 
And sure enough, ere breakfast is well over, the first 
ominous big drops are seen falling. Vou cling to the 
hope that it is only a shower which will soon be over, 



2 SCIENCE PRIMERS, [introduction. 

and you go on with the preparations for the journey 
notwithstanding. But the rain shows no symptom of 
soon ceasing. The big drops come down thicker and 
faster; httle pools of water begin to form in the 
hollows of the road, and the window-panes are now 
streaming with rain. With sad hearts you have to 
give up all hope of holding. your excursion to-day. 

2. It is no doubt very tantalizing to be disappointed 
in this way when the promised pleasure was on the 
very point of becoming yours. But let us see if we 
cannot derive some compensation even from the bad 
weather. Late in the afternoon the sky clears a little, 
and the rain ceases. You are glad to get outside 
again, and so we all sally forth for a walk. Streams of 
muddy water are still coursing along the sloping road- 
way. If you will let me be your guide, I would advise 
that we should take our walk by the neighbouring 
river. We wend our way by wet paths and green 
lanes, where every hedgerow is still dripping with 
moisture, until we gain the bridge, and see the river 
right beneath us. What a change this one day's heavy 
rain has made ! Yesterday you could almost count 
the stones in the channel, so small and clear was the 
current. But look at it now ! The water fills the 
channel from bank to bank, and rolls along swiftly. 
We can watch it for a little from the bridge. As it 
rushes past, innumerable leaves and twigs are seen 
floating on its surface. Now and then a larger branch, 
or even a whole tree-trunk, comes down, tossing and 
rolling about on the flood. Sheaves of straw or hay, 
planks of wood, pieces of wooden fence, sometimes 
a poor duck, unable to struggle against the current, 
roll past us and show how the river has risen above 



INTRODUCTION.] PHYSICAL GEOGRAPHY. 



its banks and done damage to the farms higher up its 
course. 

3. We linger for a while on the bridge, watching this 
unceasing tumultuous rush of water and the constant 
variety of objects which it carries down the channel. 
You think it was perhaps almost worth while to lose 
your holiday for the sake of seeing so grand a sight as 
this angry and swollen river, roaring and rushing with 
its full burden of dark water. Now, while the scene 
is still fresh before you, ask yourselves a few simple 
questions about it, and you will find perhaps addi- 
tional reasons for not regretting the failure of the 
promised excursion. 

4. In the first place, where does all this added mass 
of water in the river come from ? You say it was the 
rain that brought it. Well, but how should it find its 
way into this broad channel ? Why does not the rain 
run off the ground without making any river at all ? 

5. But, in the second place, where does the rain 
come from ? In the early morning the sky was bright, 
then clouds appeared, and then came the rain, and you 
answer that it was the clouds which supplied the rain. 
But the clouds must have derived the water from some 
source. How is it that clouds gather rain, and let it 
descend upon the earth ? 

6. In the third place, what is it which causes the rive*- 
to rush on in one direction more than another ? When 
the water was low, and you could, perhaps, almost 
step across the channel on the stones and gravel, the 
current, small though it might be, was still quite per- 
ceptible. You saw that the water was moving along 
the channel always from the same quarter. And now 
when the channel is filled with this rolling torrent of 



4 SCIENCE PRIMERS. [introduction. 

dark water, you see that the direction of the current is 
still the same. Can yoji tell why this should be ? 

7. Again, yesterday the water was clear, to-day it 
is dark and discoloured. Take a little of this dirty- 
looking water home with you, and let it stand all night 
in a glass. To-morrow morning you will find that it 
is clear, and that a fine layer of mud has sunk to the 
bottom. It is mud, therefore, which discolours the 
swollen river. But where did this mud come from? 
Plainly, it must have something to do with the heavy 
rain and the flooded state of the stream. 

8. Well, this river, whether in shallow or in flood, is 
always moving onward in one direction, and the mud 
which it bears along is carried towards the same point 
to which the river itself is hastening. While we sit on 
the bridge watching the foaming water as it eddies 
and whirls past us, the question comes home to us 
— what becomes of all this vast quantity of water and 
mud? 

9. Remember, now, that our river is only one of 
many hundreds which flow across this country, and 
that there are thousands more in other countries 
where the same thing may be seen which we have 
been watching to-day. They are all flooded when 
heavy rains come ; they all flow downwards ; and all 
of them carry more or less mud along with them. 

10. As we walk homewards again, it will be well to 
put together some of the chief features of this day's 
experience. We have seen that sometimes the sky 
is clear and blue, with the sun shining brightly and 
warmly in it ; that sometimes clouds come across 
the sky, and that when they gather thickly rain is apt 
to fall. We have seen that a river flows ; that it is 



INTRODUCTION.] PHYSICAL GEOGRAPHY. 5 

swollen by heavy rain, and that when swollen it is apt 
to be muddy. In this way we have learnt that there 
is a close connection between the sky above us and 
the earth under our feet. In the morning, it seemed 
but a little thing that clouds should be seen gathering 
overhead ; and yet, ere evening fell, these clouds led 
by degrees to the flooding of the river, the sweeping 
down of trees, and fences, and farm produce ; and it 
might even be to the destruction of bridges, the inun- 
dation of fields and villages and towns, and a large 
destruction of human life and property. 

11. But perhaps you live in a large town and have 
no opportunity of seeing such country sights as I have 
been describing, and in that case you may naturally 
enough imagine that these things cannot have much 
interest for you. You may learn a great deal, how- 
ever, about rain and streams ^ even in the streets of a 
town. Catch a little of the rain in a plate, and you 
will find it to be so much clear water. But look at it 
as it courses along the gutters. You see how muddy 
it is. It has swept away the loose dust worn by wheels 
and feet from the stones of the street, and carried it 
into the gutters. Each gutter thus becomes like the 
flooded river. You can watch, too, how chips of 
straw, corks, bits of wood, and other loose objects 
lying in the street are borne away, very much as the 
trunks of trees are carried by the river. Even in a 
town, therefore, you can follow how changes in the sky 
lead to changes on the earth. 

12. If you think for a little, you will recall m.any 
other illustrations of the way in which the common 
things of everyday life are connected together. As far 
back as you can remember, you have been familiar with 



6 SCIENCE PRIMERS, [introduction. 

such things as sunshine, clouds, wind, rain, rivers, frost, 
and snow, and they have grown so commonplace that 
you never think of considering about them. You cannot 
imagine them, perhaps, as in any way different from 
what they are ; they seem, indeed, so natural and so 
necessary that you may even be surprised when anyone 
asks you to give a reason for them. But if you had 
lived all your lives in a country where no rain ever 
fell, and if you were to be brought to such a 
country as this, and were to see such a storm of rain 
as you have been watching to-day, would it not be 
very strange to you, and would you not naturally 
enough begin to ask the' meaning of it ? Or suppose 
that a boy from some very warm part of the world 
were to visit this country in winter, and to see for the 
lirst time snow fall, and the rivers solidly frozen-over, 
would you be surprised if he showed great astonish- 
ment ? If he asked you to tell him what snow is, and 
why the ground is so hard, and the air so cold, why 
the streams no longer flow, but have become crusted 
with ice — could you answer his questions ? 

13. And yet these questions relate to ver}' common, 
everyday things. If you think about them, you will 
learn, perhaps, that the answers are not quite so easily 
found as you had imagined. Do not suppose that 
because a thing is common, it can have no interest for 
you. There is really nothing so common as not to 
deserve your attention, and which will not reward you 
for your pains. 

14. In the following pages I propose to ask you to 
look with me at some of these common things. You 
must not think, however, that it is my wish merely to 
set you certain lessons which you have to learn, and 



INTRODUCTION.] PHYSICAL GEOGRAPHY. 7 

to give you some rudiments of knowledge which you 
must commit to memory. I would fain have you not 
to be content with what is said in this little book, or 
in other books, whether small or great, but rather to 
get into the habit of using your own eyes and seeing 
for yourselves what takes place in this wonderful 
world of ours. All round you there is abundant 
material for this most delightful inquiry. No excur- 
sion you ever made in pursuit of mere enjoyment and 
adventure by river, heath, or hill, could give you more 
hearty pleasure than a ramble with eyes and ears alike 
open to note the lessons to be learnt from every day 
and from every landscape. Remember that besides 
the printed books which you use at home, or at school, 
there is the great book of Nature, wherein each of us, 
young and old, may read, and go on reading all tlirough 
life without exhausting even a small part of what it 
has to teach us. 

15. It is this great book — Air, Earth, and Sea — 
which I would have you look into. Do not be content 
with merely noticing that such and such events take 
place. For instance, to return to our walk to the 
flooded river; do not let a fact such as a storm or 
a flood pass without trying to find out something 
about it. Get into the habit of asking Nature ques- 
tions, as we did in the course of our homeward walk. 
Never rest until you get at the reasons for what you 
notice going on around you. In this way even the 
commonest things will come to wear a new interest for 
you. AVherever you go there will be something for 
you to notice ; something that will 'serve to increase 
the pleasure which the landscape would otherwise 
afford. You will thus learn to use your eyes quickly 
9 



8 SCIENCE PRIMERS. [shape of 

and correctly ; and this habit of observation will be of 
the utmost value to you, no matter what may be the 
path of life which lies before you. 

1 6. In the following Lessons I wish to show you what 
sort of questions you may put about some of the chief 
parts of the book of Nature, and especially about two 
of these — the Air and the Earth. Each of us should 
know something about the air we breathe and the earth 
we live upon, and about the relations between them. 
Our walk showed us a little regarding these relations 
when it enabled us to connect the destruction of fences 
and farms with the formation of clouds in the sky. 
Many other relations remain for you to find out. In 
tracing these you are really busy with science, with 
that branch of science called Physical Geography, 
which seeks to describe this earth with all the move- 
ments which are going on upon its surface. And yet 
you are not engaged in anything very difficult or un- 
interesting. You are simply watching with attentive 
eyes the changes which are continually taking place 
around you, and seeking to find out the meaning of 
these changes, and how they stand related to each 
other. 

THE SHAPE OF THE EARTH. 

17. Before observing what takes place on the surface 
of the earth, it may be well if you form a clear notion 
about the shape of the whole earth as a mass, and if 
you fix in your minds some of the great leading 
features of the connection between the earth and 
the sun. 

18. When you stand in the middle of a broad flat 
country, or look out upon the wide sea, it seems to 



THE EARTH.] PHYSICAL GEOGRAPHY. g 

you as if this world on which we live and move were 
a great plain, to the edge of which you would come if 
you went far enough onward. This is the first notion 
we all have as children. It was also the firm belief 
of mankind in early times. The sun and moon were 
then thought to rise and set only for the use of people 
here ; and the sky, with all its stars, was looked upon 
as a great crystal dome covering and resting upon 
the earth. 

19. But you can easily prove to yourselves that the 
eye is deceived about the flatness of the earth, and 
that what seems quite level is in reality curved. In a 
wide level country, such as many parts of the midland 
and eastern counties of England, you cannot see trees 
and houses farther away than some four or five miles. If 
you climb to the top of a church tower, you find many 
objects come into sight which you could not have 
seen from the ground. And if there should happen 
to be a range of hills in the neighbourhood, you would 
note from their tops a still larger number of points 
which before were hidden. The higher you climb 
above the ground, therefore, the further you can see. 

20. Again : suppose you were at the bottom of a tall 
sea-cliff, and on looking out to sea were to note the 
sails of a distant ship. If you mounted to the top of 
the cliff, you might see not only the sails, but the 
whole vessel, and your eye would probably pick out 
ships still further away, appearing as mere specks 
along the line of meeting between sea and sky, and 
which you could not see at all from the beach. 

21. Suppose further, that you were to sit down on the 
top of that cliff, and watch these vessels for a time. 
Some of them, which at first were so far away that 



lO 



SCIENCE PRIMERS. 



[shape of 



they could hardly be seen, would probably seem to 
grow bigger and clearer. You would begin to make 
out the tops of the masts and sails ; by and by the 
rest of the sails would appear, until at last the hulls 
too came into sight. These vessels would seem' to 
you to have sailed up over what used to be thought 
the edge of the world. 




Fig. I. — Disappearance of a Ship at Sea owing to the curved surface of 

the Earth. 



22. On the other hand, some of the ships which were 
near you at first will gradually sail away towards the 
same distant parts. Their hulls will dip .down into 
the sea, as it were ; then the sails will slowly sink, and 
in the end all trace of the vessels will have vanished. 

23. Now, in making these observations, you will have 
gathered facts which prove that the world we live in 
is not a flat plain, but has a curved surface, or in other 
words is a globe. To use your eyes in this way, and 
seek out the meaning of that which you see, would 
neither be a hard nor a dull task ; and yet you would 
really be engaged in what is called observational 
science. When you watch how the ships at sea appear 



THE EARTH.] PHYSICAL GEOGRAPHY. ii 

to you as they come and go, you observe facts. When 
you put the facts together, and reason out their con- 
nection and meaning, and find that they prove the 
roundness of the earth, you make an induction or 
inference from them. Now it is this union of obser- 
vation and induction which makes science. 

24. You may observe, then, and prove that the old 
and natural-enouerh notion about the flatness of the 
earih is quite untrue ; and that, flat as the sea and 
land may appear, they are only parts of a great curve. 
If youwere to set sail from England, and keep sailing 
on in the same general direction without turning back, 
you would in the end come to England again. You 
would sail round the world, and prove it to be actually 
a globe. Now, this has often been done. Many voy- 
ages have been made round the world, and, instead 
of coming to its edge, the voyagers, or " circumnavi- 
gators," as they are called, have always found the 
land and sea to wear the same curved surface which 
we can see for ourselves at home. 

25. Though you may find it easy enough to believe 
that the surface of the earth is part of a curve when 
you look out upon the broad sea, yet when you see 
a landscape where the ground is very uneven, such, 
for example, as a region of high mountains and deep 
valleys, you may find perhaps some difficulty in under- 
standing how it can possibly be that such an irregular 
surface can be spoken of as part of a curve. In 
reality, however, the earth is so big, that even the 
highest mountains are in comparison merely like little 
grains on the surface. It is only when the surface is 
level, as on a great plain or on the sea, that we can 
usually judge by the eye as to the real form of the 



12 



SCIENCE PRIMERS. 



[the earth. 



earth. But even in the most rugged ground the curve 
is there, though we may fail to notice it. 

26. But the curve, after all, is a very gentle one. 
You can see the vessels at sea for many miles before 
they sink down out of sight. The fact that the curve 
is so gentle shows that the circle of which it forms 




Fig. 2.— The Earth and Moon as they would appear seen from the Sun. 

part must be of great size. Now, it has been nieasured 
by astronomers, and found to be so big that if a rail- 
way train could go completely round the earth at a 
rate of thirty miles an hour without stoppage, it would 
take more than a month to complete the circuit. 



DAY, ETC.] PHYSICAL GEOGRAPHY. 13 



DAY AND NIGHT. 

27. Day by day, as far back as you can remember, 
you have been accustomed to see the sun travel across 
the sky. Night after njght, when the air has been free 
from cloud, you have seen the moon and stars sailing 
slowly overhead. You cannot be more confident of 
anything than you are that the sun will appear again 
to-morrow, and move on from year to year as it has 
done in the past. You have seen that a slow, regular, 
and unceasing motion seems to be going on all round 
the earth. Have you ever wondered what can be the 
cause of this motion ? 

28. When the sun shines it is warm, when clouds 
obscure the sky the air is more chilly, and at night, 
when the sun does not shine at all, we feel a sensation 
of cold. Again : by day the sky is filled with light, 
but when the sun sinks in the west darkness begins. 
You see from this that we depend upon the sun for 
light and heat. It is evident that we cannot properly 
understand what takes place upon the earth until we 
learn something about the relations of the earth to 
the sun. 

29. Perhaps your first impression has been like that of 
mankind in general long ago. They believed the earth 
to remain as the fixed central point of the universe, 
round which sun, moon, and stars were ceaselessly 
revolving. To this day we speak of these heavenly 
bodies as rising and setting, as if we still regarded 
them as performing a journey round the earth. 

30. But instead of being the centre of the universe 
our earth is in reality only one of a number of heavenly 



14 SCIENCE PRIMERS. [day and 

bodies which travel unceasingly round the sun. The 
sun is the great central hot mass which warms and 
lights the earth, and round which the earth is con- 
tinually circling. 

31. The succession of day and night seems to be 
owing to the movements of the sun, but in reality it is 
caused by the turning or rotation of the earth itself. 
You can readily illustrate this. Set a humming-top 
spinning as rapidly as you can. It seems to stand 
for a while motionless upon its point, but actually it 
is rotating with great rapidity. Imagine a line passing 
straight up from the point below, to the top of the 
stalk above. Every part of the top is spinning round 
this central line, which is called the axis of rota- 
tion. In the same kind of way the earth is spinning 
rapidly on its axis. 

32. Again : take an ordinary school-globe, and place 
a lighted candle a few feet from it, in a line with the 
brass circle. You can make the globe turn round on 
its axis. Whether it is allowed to remain at rest or 
is sent spinning round rapidly, the half of it next the 
candle is lighted, and the other half away from the 
candle is in shade. When it is at rest, the places 
marked on one side remain in the light, while those 
on the opposite side remain in the dark. As you turn 
it round, each place in succession is brought round to 
the light, and carried on into the shade again. And 
while the candle remains unmoved, the rotation of 
the globe brings alternate light and darkness to each 
part of its surface. 

33. Instead of the little school-globe in this illustra- 
tion, imagine our earth, and in place of the feeble 
candle, the great sun, and you will see how the rotation 



NIGHT.] PHYSICAL GEOGRAPHY. 15 

of the earth on its axis must bring alternate light and 
darkness to every country. 

34. You must not suppose that there is any actual 
rod passing through the earth to form the axis round 
which it turns. The axis is only an imaginary line, 
and the two opposite points where it reaches the sur- 
face, and where the ends of the rod would come out 
were the axis an actual visible thing, are called the 
North Pole and the South Pole. They are 
represented by the two little points by which the 
school-globe is fixed in its place. 

35. Round this axis the earth spins once in every 
twenty-four hours. All this time the sun is shining 
steadily and fixedly in the sky. But only those parts 
of the earth can catch his light which happen at any 
moment to be looking towards him. There must 
always be a bright side and a dark side, just as there 
was a bright side and a dark side when you placed 
the globe opposite to the candle. Now you can 
easily see that if there were no motion in the earth, 
half of its surface would never see the light at all, 
while the other half would never be in darkness. But 
since it rotates, every part is alternately illuminated 
and shaded. When we are catching the sun's light, 
we have Day ; when we are on the dark side, we 
have Night. 

2i(). The sun seems to move from east to west. The 
real movement of the earth is necessarily just the 
reverse of this, viz. from west to east In the morn- 
ing we are carried round into the sunlight, which 
appears in the east. Gradually the sun seems to 
climb the sky until we are brought directly opposite 
to him at noon, and gradually he sinks again to set in 



1 6 SCIENCE PRIMERS. [the 



the west, as the earth in its constant rotation bears us 
round once more into the dark. Even at night, how- 
ever, we can still trace the movement of the earth by 
the way in which the stars one by one rise and set, until 
their lesser lights are quenched in the returning light 
of another day. 

37 . All the time that the earth is rotating on its axis it 
is circling or revolving round the sun. This motion 
is called the revolution of the earth in its orbit. 
To go completely round the sun, the earth has to travel 
over so wide a circle or orbit, that it takes rather 
more than three hundred and sixty-five days to per- 
form the journey, even though it is rushing along at 
an average speed of about nineteen miles in a second. 

2i^. By the motion of rotation, time is divided into 
days and nights, by that of revolution it is marked off 
into years. So that in this way the earth is our great 
time-keeper. 

THE AIR. 

I. What the Air is made of. 

39. When we begin to look attentively at the world 
around us, one of the first things to set us thinking 
is the air. We do not see it, and yet it is present 
wherever we may go. At one time it blows upon us 
in a gentle breeze, at another it sweeps along in a 
fierce storm. What is this air? 

40. Although invisible, it is yet a real, material sub- 
stance. When you swing your arm rapidly up and down 
you feel the air offering a resistance to the hand. The 
air is something which you can feel, though you cannot 
see it. You breathe it every moment. You cannot get 



AIR.] PHYSICAL GEOGRAPHY. 17 

away from it, for it completely surrounds the earth. 
To this outer envelope of air, the name of Atmo- 
sphere is given. 

41. From the experiments explained in the Chemis- 
try Primer (Art. 9) you learn that the air is not a simple 
substance, but a mixture of two invisible gases, calle^i 
nitrogen and oxygen. But besides these chief ingre- 
dients, it contains also small quantities of other sub- 
stances ; some of which are visible, others invisible. If 
you close the shutters of a room, and let the sunlight 
stream through only one chink or hole into the room, 
you see some of the visible particles of the air. Hun- 
dreds of little motes or specks of dust cross the beam 
of light which makes them visible against the sur- 
rounding darkness, though they disappear in full day- 
light. But it is the invisible parts of the air which 
are of chief importance j and among them there are 
two which you must especially remember — the vapour 
of water and carbonic acid gas. You will soon 
come to see why it is needful for you to distinguish 
these. 

42. Now what is this vapour of water? You will 
understand its nature if you watch what takes place 
when a kettle boils. From the mouth of the spout a 
stream of white cloud comes out into the air. It is in 
continual motion; its outer parts somehow or other 
disappear, but as fast as they do so they are suppHed 
by fresh materials from the kettle. The water in the 
kettle is all the while growing less, until at last, if you 
do not replenish it, the whole will be boiled away, and 
the kettle left quite dry. What has become of all 
the water? You have changed it into vapour. It is 
not destroyed or lost in any way, it has only passed 



1 8 SCIENCE PRIMERS. [the 



from one state into another, from the liquid into the 
gaseous form, and is now dissolved in the air. 

43. Now the air always contains more or less vapour 
of water, though you do not see it, so long as it remains 
in the state of vapour. It gives rise to clouds, mist, 
rain, and snow. If it were taken out of the air, every- 
thing would be dried up on the land, and life would 
be impossible. As you learn more and more of the 
changes which take place from day to day around you, 
you will come to see that this vapour of water plays 
a main part in them all. 

44. Carbonic acid gas is also one of the invisible 
substances of the atmosphere, of which, though it 
forms no more than four parts in every ten thousand, 
yet it constitutes an important ingredient. You will 
understand how important it is when you are told that, 
from this carbonic acid in the air, all the plants which 
you see growing upon the land extract nearly the whole 
of their solid substance (see Chemistry Primer, Art. 
11). When a plant dies and decays, the carbonic acid 
is restored to the air again. On the other hand, plants 
are largely eaten by animals, and help to form the 
framework of their bodies. Animals in breathing 
give out carbonic acid gas ; and when they die, and 
their bodies decay, the same substance is again re- 
stored to the atmosphere. Hence the carbonic acid 
of the air is used to build up the structure both of 
plants and animals, and is given back again when 
these living things cease to live. There is a continual 
coming and going of this material between the air 
and the animal and vegetable kingdoms (see Che- 
mistry Primer, Art. 13). 



AIR.] PHYSICAL GEOGRAPHY. 19 

II. The Warming and Cooling of the Air. 

45. You know that though you cannot see the air you 
jcan feel it when it moves. A light breeze, or a strong 
gale, can be just as little seen by the eye as still air; 
and yet we readily feel their motion. But even when 
the air is still it can make itself sensible in another 
way, viz. by its temperature (see Physics Primer^ 
Art. 51). For air, like common visible things, can be 
warmed and cooled. 

46. This warming ani5 cooling of the air is well illus- 
trated by what takes place in a dwelling-house. If you 
pass out of a warm room, on a winter's day, into ^^he 
open air when there is no wind, you feel a sensation 
of cold. Whence does this sensation come? Not 
from anything you can see, for your feet, though resting 
on the frozen ground, are protected by leather, and 
do not yet feel the cold. It is the air which is cold, 
and which encircles you on all sides, and robs you of 
your heat ; while at the same time you are giving off 
or radiating heat from your skin into the air (see 
Physics Primer, Art. 67). On the other hand, if, after 
standing a while in the chilly winter air you return 
into the room again, you feel a sensation of pleasant 
warmth. Here, again, the feeling does not come from 
any visible object, but from the invisible air which 
touches every part of your skin, and is thus robbed 
of its heat by you. 

47. Air, then, may sometimes be warm and some- 
times cold, and yet still remain quite invisible. By 
means of the thermometer (which is explained in the 
Physics Primer, Art. 51), we can measure slight 
changes of heat and cold, which even the most 
sensitive skin would fail to detect. 

3 



20 SCIENCE PRIMERS. ^ ■ [the 

48. Now, how is it that the atmosphere should 
sometimes be warm and sometimes cold ? Where 
does the lieat come from ? and how does the air take 
it up ? 

49. Let us return again to the illustration of the 
house. In winter, when the air is keen and .frosty 
outside, it is warm and pleasant indoors, because fires 
are there kept burning. The burning of coal and 
wood produces heat, and the heat thus given out 
warms the air. Hence it is by the giving off or 
radiation of the heat from some burning substance 
that the air of our houses is made warmer than the air 
outside. 

50. Now, it is really by radiation from a heated 
body that the air outside gets its heat. In sum- 
mer, this air is sometimes far hotter than is usual in 
dwelling-houses in winter. All this heat comes from 
the sun, which is an enormous hot mass, continually 
sending out heat in all directions. 

51. But, if the sun is always pouring down heat upon 
the earth, why is the air ever cold ? Place a screen 
between you and a bright fire, and you will imme- 
diately feel that some of the heat from the fireplace 
has been cut off. When the sun is shining, expose 
your hand to its beams for a time, and then hold a 
book between the hand and the sun. At first, your 
skin was warmed ; but the moment you put it in the 
shade, it is cooled again. The book has cut off the 
heat which was passing directly from the sun to your 
hand. When the atmosphere is felt to be cold, some- 
thing has come in the way to keep the sun's heat from 
directly reaching us. 

52. Clouds cut off the direct heat of the sun. 



AIR.] 



PHYSICAL GEOGRAPHY. 



21 



You must often have noticed the change of tem- 
perature, when, after the sun has been shining 
for a time, a cloud comes between it and the 
earth. Immediately a feeling of chilliness is ex- 
perienced, which passes off as soon as the cloud has 
sailed on, and allowed the sun once more to come 
out. 

53. The air itself absorbs some of the sun's heat, 
and the greater the thickness of air through which 
that heat has to make its way, the more heat will be 




Fig. 3.— Diagram showing the influence of the varying thickness of the 
atmosphere in retarding the Sun's heat. a. Line of Sun's rays in the 
morning, b. Line of the rays at noon. c. Line of the rays at sunset. 



absorbed. Besides this, the more the rays of heat are 
slanted the weaker do they become. At noon, for 
example, the sun stands high in the sky. Its rays (as 
at B in Fig. 3) are then nearest to the vertical, and 
have also the least thickness of air to pass through 
before they reach us. As it descends in the after- 
noon, its rays get more and more slanted, and must 
also make their way through a constantly increasing 
thickness of air (as at c in the diagram). Hence the 
middle of the day is much warmer than morning or 
evening. 



22 SCIENCE PRIMERS. [the 



1 

54. At night, when the sun no longer shines, its 
heat does not directly wa.rm the part of the earth in 
shadow. That part not only receives no heat from it, 
but even radiates its heat out into the cold sky (see 
Art. 59). Hence night is much colder than day. 

55. Then, again, in summer the sun at noon shines 
m.Qch higher in the sky with us, or more directly over- 
head, than in winter. Its heat comes down less 
obliquely and has less depth of air to pass through, 
and hence is much more felt than in winter, when, 
as you know, the sun in our part of the world never 
rises high even at midday. 

56. From all this it is evident that we get our sup- 
plies of heat from the sun, and that anything coming 
between us and the sun serves to interrupt this heat 
and give us the? sensation of cold. 

57. Still, if we were dependent for our warmth upon 
the direct heat of the sun alone, we should be warm 
only when the sun shines. A cloudy day would be 
an extremely cold one, and every night as intensely 
frosty as it ever is in winter. Yet such is not the 
case. Cloudy days are often quite warm ; while we are 
all aware that the nights are by no means always very 
cold. There must be some way in which the sun's 
heat is stored up, so that it can be felt even when he 
is not shining. 

58. Let us again have recourse to our first illus- 
tration. If you place the back of a chair opposite to 
the fire, you will find that it gets so hot that you can 
hardly touch it. Remove the chair to a distant part 
of the room, and it quickly cools. Hence a part of 
the heat from the fire has been absorbed by the 
wood, and again given out. 



AIR.] PHYSICAL GEOGRAPHY. 23 

59. In like manner in summer the ground gets 
warmed \ in some parts, indeed, becoming even so hot 
at times that we can hardly keep the hand upon it. In 
hot countries this is felt much more than in Britain. 
Soil and stones absorb heat readily, that is to say, soon 
get heated, and they soon cool again. When they have 
been warmed by the sun, the air gets warmed by con- 
tact with them, and keeps its heat longer than they 
do; so that even when at night the soil and stones 
have become ice-cold, the air a little above is njbt so 
chilly. On the other hand, when the surface of the 
ground is cold, it cools the air next it. The ground 
parts easily with its heat, and a vast amount of heat is 
in this way radiated at night from the earth outward 
into the cold starry space. Much more heat, however, 
would be lost from this cause did not the abundant 
aqueous vapour of the atmosphere (Art. 43) absorb 
part of it, and act as a kind of screen to retard the 
radiation. This is the reason why in hot climates, 
where the air is very dry — that is, contains a small 
proportion of the vapour of water — the nights are re- 
latively colder than they are in other countries where 
the air is moister. In like manner, clouds serve to 
keep heat from escaping ; and hence it is that cloudy 
nights are not so cold as those which are clear and 
starry. 

60. The atmosphere, then, is heated or cooled ac- 
cording as it lies upon a warm or cold part of the 
earth's surface ; and, by means of its aqueous vapour, 
it serves to store up and distribute this heat, keeping 
the earth from such extremes of climate as would 
otherwise prevail. 



24 SCIENCE PRLMERS. [the 

III. What happens when Air is warmed 
or cooled — Wind. 

6 1. The air lying next to a hot surface is heated; 
the air touching a .cold surface is cooled. And upon 
such differences of temperature in the air the formation 
of winds depends. 

62. Hot or warm air is lighter than cold air. You 
have learnt how heat expands bodies (Physics Primer, 
Art. 49). It is this expansion of air, or the separation 
of its particles farther from each other, which makes 
it less- dense or heavy than cold air, where the particles 
lie more closely together. As a consequence of this 
difference of density, the light warm air rises, and the 
heavy cold air sinks. You can easily satisfy yourselves 
of this by experiment. Take a' poker, and heat the 
end of it in the fire until it is red-hot. Withdraw it, 
and gently bring some small bits of very light paper 
or some other light substance a few inches above the 
heated surface. The bits of paper will be at once 
carried up into the air. This happens because the air 
heated by the poker immediately rises, and its place 
is taken by colder air, which, on getting warmed, like- 
wise ascends. The upward currents of air grow feebler 
as the iron cools, until, when it is of the same tem- 
perature as the air around, they cease. 

d'l. This is the principle on which our fireplaces are 
constructed. The fire is not kindled on the hearth, 
for, in that case, it would not get a large enough 
draught of air underneath, and would be apt to go 
out. It is placed some way above the floor, and a 
chimney is put over it. As soon as the fire is lighted, 
the air next it gets warmed, and begins, to mount, and 
the air in the room is drawn in from below to take the 



AIR.] PHYSICAL GEOGRAPHY. 25 

place of that which rises. All the air which lies above 
the burning coal gets warmer and lighter ; it therefore 
flows up the chimney, carr}dng with it the smoke and 
gases. You will understand that though a bright 
blazing fire is a pleasant sight in winter, we do not 
get all the heat which it gives out. On the contrary, 
a great deal of the heat goes up the chimney ; and, 
except in so far as it warms the walls, passes away and 
warms the outer air. 

64. What happens in a small way m our houses takes 
place on a far grander scale in nature. As already 
pointed out (Art. 50), the sun is the great source of 
heat which warms and lightens our globe. While the 
heat of the sun is passing through the air, it does very 
little in the way of warming it. The heat goes through 
the air, and warms the surface of the earth. You knov/ 
that in summer the direct rays of the sun are hot 
enough to bum your face, and yet, if you put even a 
thin sheet of paper over your head, enough to cut off 
these rays, the sensation of burning heat at once goes 
off, although the same air is playing about you all the 
time. 

65. Both land and water are heated by the sun's 
rays, and the same change in the air then takes place 
which we find also at our firesides. The layer of air 
next the warmed earth becomes itself warmed. As it 
thereby grows lighter it ascends, and its place is taken 
by colder air, which flows in from the neighbourhood 
to take its place. This flowing in of air is Wind. 

66. It is easy for you now and then to watch how 
wind arises. Suppose, for instance, that during the 
summer you spend some time at the sea-coast. In the 
morning and early part of the day a gentle wind will 



26 SCIENCE PRIMERS. [the 



often be noticed, blowing from the land out to sea. 
As the day advances, and the heat increases, this wind 
dies away. But after a while, when the day is beginning 
to sink towards evening, another breeze may be noticed 
springing up from the opposite quarter, and blowing 
with a delicious coolness from the sea to the land. 
These breezes are the result of the unequal heating 
and cooling of the sea and land. 

67. Let us understand how this takes place. On a 
hot day you find that stones, soil, or other parts of the 
land get very warm under the sun's rays ; yet if you 
bathe in the sea at that time you feel its waters to be 
pleasantly cool. This shows that the land becomes 
more quickly hot than the sea. After such a hot day 
you will find that at night the surface of the land 
becomes much colder than the sea, because it parts 
with its heat sooner than the sea does. By day the 
hot land heats the air above it, and makes it lighter, 
so that it ascends ; while the cooler and heavier air 
lying on the sea flows landward as a cool and re- 
freshing sea-breeze. By night this state of things 
is just reversed ; for then the air which lies on the 
chilled land being colder and heavier than that which 
covers the warmer sea, flows seaward as a cool land- 
breeze. 

6%. Take a school-globe, and notice some of the 
lines which are drawn round it. Midway between the 
two poles you will notice a line running round the most 
projecting part of the globe. This line is called the 
equator. It divides the globe, as you see, into two 
halves or hemispheres. Now, over the parts of the 
earth which this line traverses, and for some way on 
either side, the sun shines with intense heat all the 



AIR.] PHYSICAL GEOGRAPHY. 27 

year round. The air is constantly heated to a high 
degree, and streams upwards in ascending currents. 
But just as the hot air along this central belt mounts 
up into the higher regions of the atmosphere, the 
cooler air from north and south flows in along the 
surface to supply its place. This constant streaming 
of air into the equatorial regions forms what are known 
as the Trade Winds. The steadiness of these winds, | 
and the w^ay in which they may be counted upon in 
navigation, led long ago to their being called by their 
present name. 

69. In our country the winds are by no means so 
regular and constant. If you look at the map, and 
mark the position of Britain upon the surface of the 
earth, you will readily notice some obvious reasons 
why our winds should be variable. To the west lies 
the wide Atlantic Ocean ; to the east, beyond the 
narrow and shallow North Sea, stretches the vast con- 
tinental mass of Europe and Asia. Seas and lands 
much colder than ours lie to the north ; others much 
warmer than ours spread to the south. So that, with 
so variable a surface receiving the sun's heat, we may 
be quite prepared to find that sometimes a warm wind 
blows from one quarter, and sometimes a cold wind 
from another. 

IV. The Vapour in the Air. Evaporation 
and Condensation. 

70. One of the most important ingredients in the 
air was stated in Art. 41, to be the vapour of water. 
Let us try to see, first of all, how it gets into and out 
of the air. And in this case, as before, you will find 
that great questions in science often admit of being 



28 SCIENCE PRIMERS. [the 

simply and readily illustrated by the most familiar 
things. 

71. In a warm room, where a good fire has been 
burning all day, and a number of people have been 
gathered together, you might suppose that the air must 
be tolerably dry. But bring a tumbler of ice-cold 
water into the room, and mark what happens to it. 
You will see the outside of the glass immediately 
covered with a fine film of mist. In a little while 
minute drops of water will form out of this film, and 
will go on growing, until, p( rhaps, some of them unite 
and trickle down the side of the tumbler. 

72. You may have noticed, too, that on very cold 
nights the windows of sitting-rooms or crowded public 
halls are apt to be found streaming with water on the 
inside. 

73. Now, in such cases, where does the moisture 
come from ? Certainly not out of the glass. It is 
derived from the vapour of water present in the air. 
This word vapour is often used to describe some kind 
of visible mist or fog. But these visible forms of mois- 
ture are not properly vapour in the sense in which the 
term is used in science. The aqueous vapour of the 
air is always invisible, even when the air is saturated 
with it, and only when it passes back into the state of 
water do you actually see anything. 

74. When the invisible vapour dissolved in the 
air becomes visible, as in mists, clouds, dew, or rain, 
it is said to be condensed, 'and this process of 
liquefaction is called condensation. 

75. The quantity of vapour which the air can 
contain varies according to temperature, warm air 
being able to hold more than cold air. You can show 



AIR.] PHYSICAL GEOGRAPHY. 29 

this in a simple way. In breathing you exhale at each 
breath a quantity of aqueous vapour ; when the air is 
warm, this invisible vapour, as soon as it escapes from 
you, mixes with the outer air, and is kept dissolved 
there. But if you cool the breath as it leaves your 
mouth, the vapour is at once condensed into visible 
moisture. Take a mirror, for example, or any other 
cold surface, and breathe on it ; the vapour from your 
lungs at once shows itself in a film of mist upon the 
glass, because the air in contact with the cold surface 
is chilled and cannot hold so much vapour, part of 
which is condensed. During winter you do not need 
a mirror to make the vapour of the breath visible, for 
the cold air around you at once condenses this vapour 
as it comes from the mouth, and forms the fine cloud 
or mist which appears with each breath that you 
exhale. 

76. As the air is cooled, its power of retaimng 
vapour diminishes. When it becomes colder than the 
temperature at which it is able to keep its supply of 
vapour dissolved, the excess of vapour is condensed 
and becomes visible. The temperature at which this 
takes place is the point of saturation, or Dew-point 
(see Art. 85). 

77. Perhaps you may ask how it is that the vapour 
so universally present gets into the atmosphere, 
and where it comes from. If you pour a little water 
into a plate, and set it down in the open air, you 
will note, in the course of a day or two, that the water 
has sensibly diminished. The air has drunk up part 
of it, and will drink up the whole, if the water \z 
allowed to stand long enough. What takes place 
from a small quantity of water goes on from every 



o 



o ^ SCIENCE PRIMERS. [the 



surface of water on the face of the earth, from every 
brook and river and lake, and from the great sea 
itself. Water is constantly passing off into vapour 
which is received and retained by the air. This pro- 
cess is called Evaporation, and the water which 
passes off into vapour is said to evaporate. 

78. Since warm air can hold more vapour than cold 
air, evaporation must be more vigorous in sunshine 
than at night, and during summer than during winter. 
You have often noticed a great difference in the rate 
at which wet roads will dry up. When the sun shines 
warmly upon them, an hour or two may be enough to 
drive off all the moisture from them, and make them 
white and hard again. But if the weather is cold and 
dull, they may remain wet and damp for days together. 
In the one case the warm air greedily absorbs the 
vapour of the water on the roads ; in the other, the 
cold air takes up the vapour only in small quantities. 

79. Again, on a dry bracing day evaporation goes 
on rapidly, because the air has not nearly got all the 
quantity of vapour it can hold in solution. On a 
damp day, however, when the air contains about as 
much vapour as it can hold at that particular tempera- 
ture, evaporation is quite feeble, or ceases altogether. 
This varying capacity of the air for vapour is the 
reason why laundresses find so much difference be- 
tween days, in the ease with which they can have 
their clothes dried. On some days the air is busy 
drinking up vapour everywhere, and then the clothes 
dry quickly. Such is especially the case when the 
sky is clear and the wind blows, because every moment 
a fresh quantity of air comes in contact with the 
clothes, carries off some of the vapour, and passes on 



AIR.] PHYSICAL GEOGRAPHY. 31 

to make way for fresh supplies of thirsty air. On 
other days, the air can hardly hold any more vapour ; 
and the clothes are found at the end of the day to 
be almost as wet as when they were hung out in the 
morning. 

80. When water evaporates, the vapour carries away 
some of the heat of the water with it. Put a drop of 
water on the back of your hand, and let it evaporate ; 
you notice a sensation of cold, because in evaporating 
the vapour has robbed your skin of some of its heat. 
This abstracted heat is given out again into the air, 
when the vapour is condensed. 

81. You see, then, that the air contains invisible 
aqueous vapour, which though very small in quantity, 
when compared with the amount of nitrogen and 
oxygen, is yet enormous when the whole mass of the 
atmosphere is considered ; that this vapour rises from 
every water-surface over the whole earth by the pro- 
cess of evaporation, and that it is brought back again 
into the liquid form by the process of condensation. 

V. Dew, Mist, Clouds. 

82. After sunset, when the sky is clear, you know 
that the grass gets wet with dew. In the morning you 
may see mists hanging over woods, and streams, and 
hills, and gradually melting away as the sun mounts 
in the sky. At all times of the year you may watch 
how clouds form and dissolve, and form again, ever 
changing their size and shape as they move through 
the air. Now these are all examples of the conden- 
sation of vapour. Let us see how the process takes 
place. 

^l. Condensation, as we have seen (Art. 76), results 
4 



32 • SCIENCE PRIMERS. [the 

from a cooling of the air. When vapour is condensed, 
it does not at once take the form of running water. 
The cold glass brought into the warm room has first 
a fine film of mist formed upon it, and then by 
degrees the clear drops of water come. In reality 
mist is made up of exceedingly minute particles of 
water, and it is the running together of these which 
makes the larger drops. So in nature on the great scale, 
when condensation occurs the vapour first appears 
as a fine mist. This is always the result of cooling ; 
so that, whenever you see a mist or cloud forming, 
you may conclude that the air in which it lies is being 
cooled. 

84. Dew. — This name is given to the wetness 
which we notice appearing in the evening or at iiight 
upon grass, leaves, or stones, or even sometimes on 
our hair. In the morning you have, no doubt, often 
watched tlie Httle dewdrops sparkling upon the foliage 
and the delicate threads of gossamer. Now this wet- 
ness does not come out of the leaves or stones, nor 
out of your hair. It is all derived from the air by 
condensation, exactly as we saw the film of mist form 
upon the cold tumbler in the warm moist air of a 
room. In fact, that film of mist was really dew, and 
all dew is formed in the same way, and from the same 
cause. 

85. At night, when the sky is clear, the earth radi- 
ates heat rapidly ; that is to say, it gives off into cold 
space a great part of the heat which it has received 
from the sun during the day (Art. 59). Its surface 
consequently becomes cold, as you may have felt 
when you put yOur hand upon leaves or stones after 
nightfall. The layer of air next the cooled ground is 



AIR.] PHYSICAL GEOGRAPHY. 33 

chilled below its point of condensation, and the excess 
of vapour is deposited as dew upon the grass, twigs, 
stones, and other objects. Hence it is that the 
temperature at which this condensation begins to 
take place is called the Dew-point (Art. 76). 

Zd. Mist and Fog. — Another way in which a cold 
surface of the earth may produce condensation is 
shown by what takes place among mountains. When 
a warm moist wind blows upon a chill mountain top, 
the air is cooled, and its vapour becomes visible 
in the form of a mist or cloud. You can often see 
that the cloud is quite solitary, and even shapes itself 
to the form of the ground, as if it were a sort of 
fleecy cap drawn down over the mountain's head. 
This is often well marked in the morning. As day 
advances, the ground, warmed by the sun, no longer 
cools the air, and hence the mist is gradually re- 
absorbed into the atmosphere. But by and by, at the 
coming on of night, when the ground is once more 
cooled by radiation, if there should be vapour enough 
in the air, the mist will re-form, and the mountain 
put on his cap again. 

87. Cold air, as well as cold ground, condenses the 
vapour of warmer air. If you watch what goes on 
along the course of a river, you will often see exam- 
ples of this kind of condensation. The ground on 
either side of the river parts with its heat after 
sun-down sooner than the river itself does, and con- 
sequently cools the air above it more than the air 
above the river is cooled. So when this colder air 
from either side moves over to take the place of the 
warmer damp air lying on and rising from the river, 
condensation ensues in the form of the mist or river- 



34 SCIENCE PRIMERS. [the 

fog, which so commonly hangs at night and early 
morning over streams. 

88. Clouds. — It is not on the ground, however, 
but up in the air that the chief condensation of vapour 
takes place. No feature of everyday occurrence is 
more familiar to you than the clouds, which are the 
result of this condensation. A cloud is merely a mist 
formed by the cooling of warm moist air when it 
loses its heat from any cause, such as expansion during 
ascent, or contact with currents of cooler air. If you 
watch what goes on in the sky, you may often see 
clouds in the act of forming. At first a little flake 
of white appears. By degrees this grows larger, and 
other cloudlets arise and flock together, until at last 
the sky is quite overcast with heavy clouds, and rain 
begins to fall. The vapour which is thus condensed 
in the air has all been obtained by the evaporation 
of the water on the earth's surface. It rises with the 
warm air, which losing its heat as it ascends, and 
coming too in contact with colder layers of the 
atmosphere, cannot hold all its vapour, and is obliged 
to get rid of the excess, which then condenses into 
cloud. 

89. On a summer morning the sky is often free 
from cloud. As the day advances, and the earth gets 
warmed, more vapour is raised ; and as this vapour, 
borne upward by the ascending air-currents, reaches 
the higher and colder parts of the atmosphere, it is 
chilled into the white fleecy clouds which you see 
forming about midday and in the afternoon. Towards 
evening, when less evaporation takes place, the clouds 
cease to grow, and gradually lessen in size until at 
night the sky is quite clear. They have been dis- 



AIR.] PHYSICAL GEOGRAPHY. 35 

solved again by descending and coming in contact 
with the warm air nearest to the earth. Again, you 
have often noticed that clouds move across the sky. 
They are driven along by upper currents of air, and 
of course the stronger these currents are the faster do 
the clouds travel. In this way the sky is sometimes 
completely overcast with clouds which have come 
from a distance. By watching these comings and 
goings of the clouds, you see how the state of the 
vapour in the atmosphere continually changes. At 
one time it is condensed into clouds, at another 
time evaporated and made invisible by the varying 
currents of the air. 

VI. Where Rain and Snow come from. 

90. You have now traced the vapour which the 
sun's heat raises from the rivers, lakes, and seas 
of the earth, and you have found it to be condensed 
again into visible form in the clouds. But the clouds 
do not remain always suspended in the sky. Some- 
times they melt away again, and are dissolved into 
invisible vapour. But they often disappear in another 
way. They let their moisture fall through the air to 
the earth, and thus give rise to rain and snow. 

91. Rain. — You are well aware that rain always 
comes from clouds in the sky. When the sky is clear 
overhead, no rain falls. Only when it gets overcast 
does the rain come. You c; n watch a dark rain- 
cloud gather itself together and discharge a heavy 
shower upon the earth. In the illustration of the 
cold glass brought into the warm room (Art. 71), 
you remember that the film of mist formed upon the 
glass was found by degrees to gather into drops, 



36 SCIENCE PRIMERS. [the 

which trickled down the cold surface. Now the mist 
on the glass and the cloud in the sky are both formed 
of minute particles of water separated by air. It is 
the running together of these particles which gives rise 
to the drops. In the one case, the drops run down 
the cold glass. In the other case, they fall as drops 
of rain through the air. Rain therefore is thus a 
further stage in the condensation of the aqueous 
vapour of the atmosphere. The minute particles of 
the cloud, as condensation proceeds, gather more 
moisture round them, until at last they form drops of 
water too heavy to hang any longer suspended in the 
air. These then fall to the earth as rain- drops. 

92. Snow. — But there is another important form 
in which the moisture of the clouds may descend to 
the surface of the earth. When the weather is cold 
enough, there fall to the ground not drops of rain, but 
flakes of snow. 

93. If you bring snow indoors, it soon melts into 
water. If you expose this water for a time it evaporates. 
Snow, water, and aqueous vapour are thus only different 
forms of the same substance. We say that water can 
exist in three forms, — the gaseous, the liquid, and the 
solid. Snow is an example of the soHd condition. 

94. On a frosty night pools of water are covered 
with a hard transparent crust of what is called Ice. 
You may break this crust into pieces, but if the cold 
continues, a new crust will soon be formed with bits 
of the old one firmly cemented in it. And the greater 
the cold the thicker will the crust be, until perhaps 
the whole of the water in the pools may become solid. 
If you take a piece of this solid substance, you find 
it to be cold, brittle, and transparent. Brought into 



AIR.] PHYSICAL GEOGRAPHY. 37 

a warm room it soon melts into water, and you may- 
drive off the water as before into vapour. Ice is the 
general name given to water when it is in the solid 
state, such forms as snow and hail being only difterent 
appearances which ice puts on. Whenever water be- 
comes colder than a certain temperature it passes into 
ice, or freezes, and this temperature is consequently 
known as the freezing-point (Physics Primer, 
Art. 51). 

95. You might suppose that ice is but a shapeless 
thing. But gather a few snowflakes, and, that they may 
not melt, examine them out of doors. When they lie 




Fig. 4. — Forms of Snowflakes. 

together in a mass they have a pure opaque whiteness, 
but in reality they are as transparent as water ; and it 
is only from the way in which they scatter the light 
from their many glistening points, that they appear 
white. To assure yourselves of this fact, carefully 
separate one or two of the flakes upon some dark 
surface (the sleeve of a coat will do well), and you 
will find that each flake is a more or less perfect star 
with six rays, formed of little needles or crystals of 
pure transparent ice. The flakes are so delicate that 
in falling through the air they are apt to be damaged 
by coming against each other. Some of their varieties 
are shown in Fig. 4. 

96. The upper layers of the atmosphere are much 



38 SCIENCE PRIMERS. [the air. 

colder than the freezmg-point of water. In the con- 
densation which takes place there, the clouds do not 
resolve themselves into rain. The vapour of the up- 
streaming currents of warm air from the earth's surface 
is condensed and frozen in these high regions, and 
passes into little crystals, which unite into flakes of 
snow. Even in summer the fine white cloudlets which 
you see floating at great heights are probably formed 
of snow. But in those countries, such as ours, where 
in winter the air even at the surface is sometimes very 
cold, the snow falls to the ground, and lies there as a 
white covering, until returning warmth melts it away. 

97. Besides rain and snow, the moisture of the air 
takes sometimes the form of Hail, which consists of 
little lumps of ice like frozen rain ; and of Sleet, which 
is partially melted snow. But rain and snow are the 
most important, and it is these two forms which we 
must follow a little further. 

98. Summary. — Before doing so, let us gather to- 
gether the sum of what has been said about the aqueous 
vapour of the air. We have learnt that, as every 
sheet of water on the face of the globe evaporates, the 
air is full of vapour ; that this vapour is condensed into 
visible form, and appears as dew, mist, and cloud. 
We have learnt further, that the vapour of which 
clouds are formed is resolved into rain and snow, 
and, in one or other of these forms, descends to the 
earth again. There is thus a circulation of water 
between the solid earth beneath and the air above. 
This circulation is as essential to the earth in making 
it a fit habitation for living things, as the circulation of 
blood is in keeping our bodies alive. It mixes and 



WATER.] PHYSICAL GEOGRAPHY. 39 

washes the air, clearing away impurities, such as those 
which rise from the chimneys of a town. It moistens 
and quickens the soil, which it renders capable of sup- 
porting vegetation. It supplies springs, brooks, and 
rivers. In short, it is the very mainspring of all the 
life of the globe. So important a part of the machinery 
of the world deserves our careful consideration. Let 
us next attend, therefore, to what becomes of the rain 
and the snow after they have been discharged from 
the air upon the surface of the earth. 

THE CIRCULATION OF WATER ON THE 

LAND. 

I. W^hat becomes of the Rain. 

99. Although air is continually evaporating water 
from the surface of the earth, and continually restoring 
it again by condensation, yet, on the whole and in 
the course of years, there seems to be no sensible 
gain or loss of water in our seas, lakes, and rivers ; 
so that the two processes of evaporation and con- 
densation balance each other. 

100. It is evident, however, that the moisture pre- 
cipitated at any moment from the air is not at once 
evaporated again. When a shower of rain falls, the 
roads are not dry the moment the shower is over. 
And when heavy rain continues for hours together, 
the whole country round may be flooded, and will, 
perhaps, remain so for days after the rain has ceased. 
The disappearance of the water is due in part to 
evaporation, but only in part. A great deal of it goes 
out of sight in other ways. 

10 1. The rain which falls upon the sea is the 



40 SCIENCE PRIMERS. [circulation 

largest part of the whole rainfall of the globe, because 
the surface of the sea is about three times greater than 
that of the land. All this rain gradually mingles with 
the salt water, and can then be no longer recognized. 
It thus helps to make up for the loss which the sea is 
always suffering by evaporation. For the sea is the 
great evaporating surface whence most of the vapour 
of the atmosphere is derived. 

1 02. On the other hand, the total amount of rain 
which falls upon all the land of the globe must be 
enormous. It has been estimated, for example, that 
about 68 cubic miles of water annually descend as rain 
even upon the surface of the British Isles, and there 
are many much more rainy regions than ours. If you 
inquire about this rain which falls upon the land, you 
will find that it does not at once disappear, but 
begins another kind of circulation. Watch what 
happens durmg a shower of rain. If the shower is 
heavy, you will notice little runnels of muddy water 
coursing down the streets or roads, or. flowing out of 
the ridges of the fields. Follow one of the runnels. 
It leads into some drain or brook, that into some 
larger stream, the stream into a river ; and the river, if 
you follow it far enough, will bring you to the sea. 
Now think of all the brooks and rivers of the world, 
where this kind of transport of water is going on, and 
you will at once see how vast must be the part of the 
rain which flows off the land into the ocean. 

103. But does the whole of the rain flow ofl" at once 
into the sea in this way ? Assuredly not, as you can 
very easily prove. Suppose that before the rain came 
the ground had been very dry, and that after the 
shower you dig up a spadeful of earth. Do you find 



OF WATER.] PHYSICAL GEOGRAPHY. 41 

the ground dry now ? No ', because some of the rain 
has soaked into the earth. And if you could dig deep 
enough, or if you were to notice what goes on when 
workmen are making a deep excavation, you would 
find that the ground underneath is not merely damp, 
but that it contains plenty of water, and that you could 
collect this water, and bring it up to the surface. 
Clearly, then, a good deal of the rain which falls 
upon the land must sink underground and gather there. 
You may think that surely the water which disappears 
in that way must be finally withdrawn from the general 
circulation which we have been tracing. When it 
sinks below the surface, how can it ever get up to the 
surface again ? 

104. Yet, if you consider for a little, you will be 
convinced that whatever becomes of it underneath it 
cannot be lost. If all the rain which sinks into the 
ground were for ever removed from the surface cir- 
culation, you will at once see that the quantity of water 
upon the earth's surface must be constantly and visibly 
diminishing. The seas must be getting narrower and 
shallower; the rivers and lakes must be drying up. 
But no such changes, so far as can be seen, are really 
taking place. The sea rolls as broadly and deeply as 
it has done for many generations past, and the lakes 
and rivers remain very much the same. So that if 
any of the water which sinks into the earth is never 
restored to the surface again, it must be so small a part 
as to make no sensible difference on the amount which 
is restored. In spite of the rain which disappears into 
the ground, the circulation of water between the air, 
the land, and the sea continues without perceptible 
diminution. 



42 SCIENCE PRIMERS. [circulation 

105. You are driven to conclude, therefore, that 
there must be some means whereby the water under- 
ground is brought back to the surface. This is done, 
as you will learn in the next section, by Springs, 
which gush out of the earth, and bring up water to 
feed the Brooks and Rivers, whereby it is borne 
into the sea. 

106. You can now answer the question, What be- 
comes of the Rain ? Most of it sinks into the earth, 
and afterwards comes out again in springs ; part of 
it is collected into brooks and rivers ; and this part, 
in so far as not evaporated, works its way over the 
land and falls at last into the sea. 

107. Here, then, are two distinct courses which the 
rainfall takes — one below ground, and one above. It 
will be most convenient to follow the underground 
portion first. 

II. Ho^v Springs are formed. 

108. In this Lesson we are to follow the course of 
that part of the rain which sinks below ground. A 
little attention to the soils and rocks which form the 
surface of a country is enough to show that they differ 
greatly from each other in hardness, and in texture or 
grain. Some are quite loose and porous, others are 
tough and close-grained. They consequently diifer 
much in the quantity of water they allow to pass 
through them. A bed of sand, for example, is per- 
vious ; that is, will let water sink through it freely, 
because the little grains of sand lie loosely together, 
touching each other only at some points, so as to leave 
empty spaces between. The water readily finds its 
way among these empty spaces. In fact, the sand- 



OF WATER.] PHYSICAL GEOGRAPHY. 43 

bed may become' a kind of sponge, quite saturated 
with the water which has filtered down from the sur- 
face. A bed of clay, on the other hand, is imper- 
vious ; it is made up of very small particles fitting 
closely to each other, and therefore offering resistance 
to the passage of water. Wherever such a bed occurs, 
it hinders the free passage of the water, which, unable 
to sink through it from above on the way down, 
or from below on the way up to the surface again, 
is kept in by the clay, and forced to find another line 
of escape. 

109. Sandy soils are dry because the rain at once 
sinks through them ; clay soils are wet because they 
retain the water, and prevent it from freely descending 
into the earth. 

no. When water from rain or melted snow sinks 
below the surface into the soil, or into rock, it does 
not remain at rest there. If you were to dig a deep 
hole in the ground, you would soon find that the water 
which Hes between the particles would begin to trickle 
out of the sides of your excavation, and gather into 
a pool in the bottom. If you baled the water out, it 
would still keep oozing from the sides, and the pool 
would ere long be filled again. This would show you 
that the underground water will readily flow into any 
open channel which it can reach. 

III. Now the rocks beneath us, besides being in 
many cases porous in their texture, such as sandstone, 
are all more or less traversed with cracks — sometimes 
mere lines, like those of a cracked window-pane, but 
sometimes wide and open clefts and tunnels. These 
numerous channels serve as passages for the under- 
ground water. Hence, although a rock may be so 
5 



44 SCIENCE PRIMERS. [circulation 

hard and close-grained that water does not soak 
through it at all, yet if that rock is plentifully supplied 
with these cracks, it may allow a large quantity of 
water to pass through. Limestone, for example, is a 
very hard rock, through the grains of which water can 
make but little way; yet it is so full of cracks or 
"joints," as they are called, and these joints are often 
so wide, that they give passage to a great deal of 
water. 

112. In hilly districts, where the surface of the 
ground has not been brought under the plough, you 
will notice that many places are marshy and wet, even 
when the weather has long been dry. The soil every- 
where around has perhaps been baked quite hard by 
the sun ; but these places remain still wet, in spite of 
the heat. Whence do they get their water ? Plainly 
not directly from the air ; for in that case the rest 
of the ground would also be damp. They get it 
not from above, but from below. It is oozing out 
of the ground ; and it is this constant outcome of 
water from below which keeps the ground wet and 
marshy. In other places you will observe that the 
water does not merely soak through the ground, 
but gives rise to a little runnel of clear water. If 
you follow such a runnel up to its source, you 
will see that it comes gushing out of the ground as 
a Spring. 

113. Springs are the natural outlets for the under- 
ground water. But you ask, why should this water 
have any outlets, and what makes it rise to the 
surface ? 

114. The following diagram (fig. 5) represents the 
way in which many rocks lie with regard to each other, 



OF WATER.] PHYSICAL GEOGRAPHY. 



45 



and in which you would meet with them if you were to 
cut a long deep trench or section beneath the surface. 
They are arranged, as you see, in flat layers or beds. 
Let us suppose that <^ is a flat layer of some imper- 
vious rock, like clay, and b another layer of a porous 
material, like sand. The rain which falls on the sur- 
face of the ground, and sinks through the upper bed, 
will be arrested by the lower one, and made either to 
gather there, or find its escape along the surface of that 




Fig. 5. -Origin of Surface Springs. 

lower bed. If a hollow or valley should have its bottom 
below the level of the line along which the water flows, 
springs will gush out along the sides of the valley, as 
shown at i" J- in the woodcut. The line of escape may 
be either, as in this case, the junction between two 
different kinds of rock, or some of the numerous 
joints already referred to. Whatever it be, the water 
cannot help flowing onward and downward, as long as 
there is any passage by which it can find its way ; and 
the rocks underneath are so full of cracks, that it has 
no difficulty in doing so. 

115. But it must happen that a great deal of the 
underground water descends far below the level of the 
valleys, and even below the level of the sea. And 
yet, though it should descend for several miles, it 
comes at last to the surface again. To realize clearly 
how this takes place, let us follow a particular drop of 
water from the time when it sinks into the earth as 



46 



SCIENCE PRIMERS. [circulation 



rain, to the time when, after a long journeying up and 
down in the bowels of the earth, it once more reaches 
the surface. It soaks through the soil together with 
other drops, and joins some feeble trickle, or some 
more ample flow of water, which works its way 
through crevices and tunnels of the rocks. It sinks in 
this way to perhaps a depth of several thousand feet 




Fig. 6. — Section of part of a district to show the origin of deep-seated 
Springs. The numerous joints in the rocks lead the water down into a main 
channel, by which it re-ascends to the surface as a spring at 5. 



until it reaches some rock through which it cannot 
readily make further way. All this while it has been fol- 
lowed by other drops, coursing after it through its wind- 
ing passage down to the same barrier at the bottom. 
The union of all these drops forms an accumulation 
of water, which is continually pressed by what is de- 
scending from the surface. Unable to work its way 
downward, the pent-up water must try to find escape 
in some other direction. By the pressure from above 



OF WATER.] PHYSICAL GEOGRAPHY. 47 



it is driven through other cracks and passages, winding 
up and down until at last it comes to the surface again. 
It breaks out there as a gushing spring (see Physics 
Primer, Art. 23). 

ri6. Thus each of the numerous springs which 
issue out of the ground is a proof that there is a cir- 
culation of water underneath, as well as upon the 
surface of the land. But besides these natural 
outlets, other proofs are afforded by the artificial 
openings made in the earth. Holes, called Wells, 
are actually dug to catch this water. Mines, pits, 
quarries, and deep excavations of any kind, are 
usually troubled with it, and need to be kept dry 
by having it pumped out. 

III. The work of Water underground. 

117. No form of water seems purer than the clear 
crystal spring as it comes bubbling out of the earth. 
Water, perfectly pure in a chemical sense, should con- 
sist only of the two elements Oxygen and Hydrogen. 
But in the water of every spring, no matter how clear 
and sparkling it may be, there is something else. If 
you take a quantity of perfectly pure water and boil it 
down, you may drive the whole of it off in steam, and 
not a vestige of anything is left behind. Rain takes 
up a little impurity from the air, yet may be regarded 
as very nearly pure water. But if you boil down a 
quantity of spring water, you find a residue of solid 
matter. Sparkling transparency is thus no guide to 
the chemical purity of the water (see Chemistry 
Primer, Arts. 20, 21). 

118. If now rain is water nearly in a state of purity, 
and if after journeying up and down underground it 



48 SCIENCE PRIMERS. [circulation 

comes out again in springs, always more or less mingled 
with other materials, it must get these materials from 
the rocks through which it travels. They are not 
visible to the eye, for they are held in what is called 
chemical solution (Chemistry Primer, Art. 23). When 
you put a few grains of salt or sugar upon a plate, and 
pour water over them, they are dissolved in the water 
and disappear. They enter into union with the water. 
You cannot see them, but you can still recognize their 
presence by the taste which they give to the water 
which holds them in solution. 

119. So water, sinking from the soil downwards, dis- 
solves a little of the substance of the subterranean 
rocks, and carries this dissolved material up to the 
surface of the ground. But you may say, salt and 
sugar are easily acted on by water, hard rocks are 
not ; how is it that the springs can get their solid im- 
purities from rocks ? 

120. You remember that one of the important ingre- 
dients in the air is carbonic acid gas, and that this sub- 
stance is both abstracted from and supplied to the air 
b)^ plants and animals (see Art. 44). In descending 
through the atmosphere rain absorbs a little air. As 
ingredients of the air, a little carbonic acid gas, particles 
of dust and soot, noxious vapours, minute organisms, 
and other substances floating in the air, are caught up 
by the descending rain, which in this way, as it were, 
washes the air, and tends to keep it much more whole- 
some than it would otherwise be. 

121. But rain not merely picks up impurities from 
the air, it gets a large addition when it reaches the 
soil. When you take up a little earth from a field 
or a garden, you may notice tiny fibres and decaying 



OF WATER.] PHYSICAL GEOGRAPHY. 49 

roots in it. It contains always more or less organic 
matter, and therefore (Art. 44) carbonic and some 
other acids. If you put some of the soil on a piece 
of iron and thrust it into the fire, you will burn off 
the organic matter, remove the carbonic acid, and 
change the colour of the soil. 

122. Armed with the carbonic acid which it gets 
from the air, and with the larger quantity which it 
abstracts from the soil, rain-water is prepared to attack 
rocks, and to eat into them in a way which pure water 
could not do (see Chemistry Primer, Experiment 28). 

123. Water containing carbonic acid has a remark- 
able effect on many rocks, even on some of the very 
hardest. It dissolves more or less of their substance, 
and removes it. When it falls for instance on chalk 
or limestone, it almost entirely dissolves and carries 
away the rock in solution, though still remaining 
clear and limpid. In countries where chalk or lime- 
stone is an abundant rock, this action of water is 
sometimes singularly shown in the way in which 
the surface of the ground is worn into hollows. In 
such districts, too, the springs are always hard \ that 
is, they contain much mineral matter in solution, 
whereas rain-water and springs which contain little im- 
purity are termed soft (Chemistry Primer, Art. 26). 

124. Many of the substances abstracted from below 
by the water of springs are useful in the life of plants 
and of animals. Lime, salt, and iron, for example, are 
all brought up in spring-water, and are all of great 
value. Lime furnishes material for the bones of 
animals, and iron supplies the colouring matter of 
their blood. We obtain, indeed, most of what we 
need of these materials from our solid food; yet spring- 



50 



SCIENCE PRIMERS. [circulation 



water, in so far as it contains them, is healthier for 
drinking and cooking than rain-water would be. 

125. As every spring throughout the world is busy 
bringing up materials of some kind to the surface, 




Fig 7. — Subterranean Channel dissolved out of Limestone-rock by Water. 

it is plain that the amount of rock dissolved and 
removed must in the end be very great. You can 
now see how there should be open channels and 



OF WATER.] PHYSICAL GEOGRAPHY. 51 

tunnels for the water underground, for the water is 
ahvays eating away a Httle of the surface over which 
it flows, thereby widening the cracks and crevices, 
and converting them by degrees into wider passages. 
In this way large caverns many feet high and many 
miles long have been formed underneath the surface 
in different parts of the world. 

IV. How the surface of the Earth crumbles 
away. 

126. When a stone building has stood for a few 
hundred years, the smoothly dressed face which its 
walls received from the mason is usually gone. The 
stones are worn into holes and furrows, the carvings 
over window and doorway are so wasted that perhaps 
you cannot make out what they were meant to repre- 
sent. This time-eaten character of old masonry is so 
familiar that one always looks for it in an old building, 
and when it is absent he at once doubts whether the 
building can really be old. 

127. Again, in the burying-ground surrounding a 
venerable church you see the tombstones more and 
more mouldered the older they are. Sometimes, 
especially in towns, the inscriptions dating from more 
than a few generations back are so greatly wasted 
that you cannot now tell whose names and virtues 
they were set up to commemorate. 

128. This crumbling away of hard stone with the 
lapse of time is a common familiar fact to you. lUit 
have you ever wondered why it should be so ? What 
makes the stone decay, and what purpose is served by 
the process ? 

129. In the case of buildings and other works of 



52 . SCIENCE PRIMERS. [circulation 

human construction the decay can be noted and 
measured, for the stones, rough and worn as they 
may be now, left the hands of the masons with 
smoothly dressed surfaces. But the decay is not 
confined to human erections. On the contrary, it 
goes on over the whole face of the world. 

130. It may seem so strange to you to be told that 
the surface of the earth is crumbling away that you 
should take every opportunity of verifying the state- 
ment. Examine all the old buildings and pieces of. 
sculpture within your reach. Look at the cliffs and 
ravines, the crags and watercourses, in your neigh- 
bourhood. At the base of each cliff you will pro- 
bably find the ground cumbered with blocks and 
heaps of lesser fragments which have fallen from 
the rocks above, and after a frosty winter you may 
even find the fresh scar whence a new mass has been 
detached to add to the pile of ruins below. 

131. After examining your own district in this way, 
you will, no doubt, find proofs that, in spite of their 
apparent steadfastness, even the hardest stones are 
really crumbling down. In short, wherever rocks are 
exposed to the air they are liable to decay. Now let 
us see how this change is brought about. 

132. First of all we must return for a moment to the 
action of carbonic acid, which has been already 
(Art. 123) described. You remember that rain-water 
abstracts a little carbonic acid from the air, and that, 
when it sinks under the earth, it is enabled by means 
of the acid to eat away some parts of the rocks 
beneath. The same action takes place with the rain, 
which rests upon or flows over the surface of the 
ground. The rain-water dissolves out little by little 



OF WATER.] PHYSICAL GEOGRAPHY. 53 

such portions of the rocks as it can remove. In the 
case of some rocks, such as Hmestone, the whole, or 
almost the whole, of the substance of the rock is 
carried away in solution. In other kinds, the portion 
dissolved is the cementing material whereby the mass 
of the rock was bound together ; so that when it is 
taken away, the rock crumbles into mere earth or 
sand, which is readily washed away by the rain. 
Hence one of the causes of the mouldering of stone 
is the action of the carbonic acid taken up by rain. 

133. In the second place, the oxygen of the por- 
tion of air contained in rain-water helps to decompose 
rocks. " When a piece of iron has been exposed for a 
time to the weather, in such a damp climate as that of 
Britain, it rusts. You know how, in the course of years, 
iron railings get quite eaten through, and how you can 
scrape the dirty yellow crust or powder from the cor- 
roded surfaces. This rust is a compound substance, 
formed by the union of oxygen with iron. It con- 
tinues to be formed as long as any of the unrusted 
iron remains, since as each crust of rust is washed oft 
a new layer of iron is laid open to the attacks of the 
oxygen. What happens to an iron railing or a steel 
knife, happens also, though not so quickly nor so 
strongly, to many rocks. They, too, rust by absorb- 
ing oxygen. A crust of corroded rock forms on their 
surface, and, when it is knocked off by the rain, a 
fresh layer of rock is reached by the ever-present and 
active oxygen. 

134. In the third place, the surface of many parts 
of the world is made to crumble down by means of 
frost. You are, no doubt, acquainted with some ot 
the eft"ects of frost. You have, probably, noticed that 



54 SCIENCE PRIMERS. [circulation 

sometimes during winter, when the cold gets very 
keen, pipes full of water burst, and jugs filled with 
water are cracked from top to bottom. The reason 
of this lies in the fact that water expands in freezing. 
Ice requires more space than the water would do if it 
remained fluid. When ice forms within a confined 
space, it exerts a great pressure on the sides of the 
vessel, or cavity, which contains it. If these sides are 
not strong enough to bear the strain to which they are 
put, they must yield, and therefore they crack (see 
Physics Primer, Art. 6i). 

135. You have now learnt how easily rain finds its 
way through soil. Even the hardest rocks are more or 
less porous, and take in some water. Hence, when 
winter comes, the ground is full of moisture ; not in 
the soil merely, but in the rocks. And so, as frost 
sets in, this pervading moisture freezes. Now, pre- 
cisely the same kind of action takes place with each 
particle of water, as in the case of the burst water- 
pipe or the cracked jar. It does not matter whether 
the water is collected into some hole or crevice, 
or is diffused between the grains of the rocks and 
the soil. When it freezes it expands, and in so doing 
tries to push asunder the walls between which it is 
confined. 

136. Hence arise some curious and interesting 
effects of frost upon the ground. If you walk along 
a road just after frost, you see that the small stones 
have been partly pushed out of their beds, and that 
the surface of the road is now a layer of fine mud. 
The frost has separated the grains of sand and clay, 
as if they had been pounded down in a mortar. 
Hence frost is of great service to the farmer in break- 



OF WATER.] PHYSICAL GEOGRAPHY. 55 

ing up the soil, and opening it out for the roots and 
fibres of plants. When a surface of rock has been 
well soaked with rain, and is then exposed to frost, 
the grains of the rock undergo the same kind of pres- 
sure from the freezing of the water in the pores 
between them. They are not so loose and open, how- 
ever, as those of the soil are, and they withstand the 
action of the frost much better. Of course, the most 
porous rocks, or those which hold most water, are 
most liable to the effects of this action. Porous 
rocks, such as sandstone, are often liable to rapid 
decav from frost. The stone has crust after crust 
peeled off from it, or its grains are loosened from 
each other and washed away by rain. 

137. Again, water freezes not only between the com- 
ponent grains, but in the numerous crevices or joints, 
as they are called, by which rocks are traversed. You 
have, perhaps, noticed that on the face of a cliff, or in 
a quarry, the rock is cut through by lines running 
more or less in an upright direction, and that by 
means of these lines the rock is split up by nature, 
and can be divided by the quarryman into large four- 
sided blocks or pillars. These lines, or joints, have 
been already (Art. in) referred to as passages for 
water in descending from the surface. You can under- 
stand that only a very little water may be admitted 
at a time into a joint. But by degrees the joint widens 
a little, and allows more water to enter. Every time 
the water freezes it tries hard to push asunder the two 
sides of the joint. After many winters, it is at last 
able to separate them a little ; then more water enters, 
and more force is exerted in freezing, until at last the 
block of rock traversed by the joint is completely split 
6 



56 



SCIENCE PRIMERS. [circulation 



up. When this takes place along the face of a cliff, 
one of the loosened parts may fall off and actually roll 
down to the bottom of the precipice. 

138. This kind of waste is represented in the ac- 
companying woodcut (Fig. 8), which gives a section of 




Fig. 8 —Waste of a Clift. 



a cliff wherein the rocks are traversed by perpendicular 
joints. These have been, widened along the front 
until large blocks have been wedged off and have 



OF WATER.] PHYSICAL GEOGRAPHY. 57 

fallen to the ground. In countries exposed to severe 
winters, the waste caused by frosts along lines of steep 
cliff is often enormous. 

139. In addition to carbonic acid, oxygen, and 
frost, there are still other influences at work by which 
the surface of the earth is made to crumble. For ex- 
ample, when, during the day, rocks are highly heated 
by strong sunshine, and then during night are rapidly 
cooled by radiation, the alternate expansion and con- 
traction caused by the extremes of temperature loosen 
the particles of the stone, causing them to crumble 
away, or even making successive crusts of the stone 
fall off. 

140. Again; rocks which are at one time well 
soaked with rain, and at another time are liable to be 
dried by the sun's rays and by wind, are apt to 
crumble away. 

141. And thus you see that from a variety of causes 
the solid rocks of the earth are liable to continual decay 
and removal. The hardest stone, as well as the softest, 
must yield in the end, and moulder down. They do 
not all indeed decay at the same rate. If you look 
more narrowly at the wall of an ancient building, you 
w411 see almost every variety in the degree of decay. 
Some of the stones are hardly worn at all, while others 
are almost wholly gone. As this takes place in a build- 
ing, you may be sure it must take place also in nature, 
and that cliffs or crags formed of one kind of stone 
will crumble down faster than others, and will do so 
in a different kind of way. 

142. If then it be true, as it is, that a general 
wasting of the surface of the land goes on, you may 
naturally ask why this should be. The world seems 



58 SCIENCE PRIMERS. [circulation 

SO fair and beautiful, that you cannot perhaps realize 
to yourselves that there should be so much decay on 
its surface. You may be even inclined at first to con- 
sider the decay as a misfortune hardly to be ex,plained. 
But instead of being a misfortune, the mouldering of 
the surface is in reality necessary to make the earth 
fit to be the dwelling-place of plants and animals. 
To it we owe the scooping out of valleys, and ravines, 
and the picturesque outlines of crags and hills. Out 
of the crumbled stones all soil is made, and on the 
formation and renewal of the soil we depend for our 
daily food. How this is brought about will be told 
in the next Lesson. 

V. What becomes of the crumbled parts 
of Rocks. How Soil is made. 

143. Take up a handful of soil from any field or 
garden, and look at it attentively. What is it made 
of? You see little pieces of crumbling stone, particles 
of sand and clay, perhaps a few vegetable fibres ; 
and the whole soil has a dark colour from the 
decayed remams of plants and animals diffused 
through it. Now let us in the present Lesson try to 
learn how these different materials have been brought 
together. 

144. We return again to the general mouldering of 
the surface of the land. The words " decay," " waste," 
and others of similar meaning, are applied to this pro- 
cess. But in reality, although the rocks may crumble 
away, and thereby grow less in size year by year, there 
is no actual loss of material to the surface of the earth. 
The substance of the rock may decay, but it is not 
destroyed. It only changes its condition and its form. 



OF WATER.] PHYSICAL GEOGRAPHY. 59 

What, then, becomes of all this material which is con- 
tinually being worn from the rocks around us ? 

145. Every drop of rain which falls upon the land 
helps to alter the surface. You have followed the 
chemical action of rain when it dissolves parts of 
rocks. It is by the constant repetition of the process, 
drop after drop, and shower after shower, for years 
together, that the rocks become so wasted and worn. 
But the rain has also a mechanical action. 

146. Watch what happens when the first pattering 
drops of a shower begin to fall upon a smooth surface 
of sand, such as that of a beach. Each drop makes 
a little dint or impression. It thus forces aside the 
grains of sand. On sloping ground, whe^e the drops 




Fig. 9. — Prints impressed on Clay or Sand by Drops of Rain. 

can run together and flow downward, they are able 
to push or carry the particles of sand or clay along. 
This is called a mechanical action ; while the actual 
solution of the particles, as you would dissolve sugar 
or salt, is a chemical action. Each drop of rain may 
act in either or both of these ways. 

147. Now you will readily see how it is that rain 
does so much in the destruction of rocks. It not only 
dissolves out some parts of them, and leaves a crum- 
bling crust on the surface, but it washes away this 
crust, and thereby exposes a fresh surface to decay. 



6o SCIENCE PRIMERS. [circulation 

There is in this way a continual pushing along of 
powdered stone over the earth's surface. Part of this 
material accumulates in hollows, and on sloping or 
level ground ; part is swept into the rivers, and carried 
away into the sea. 

148. It is this crumbled stone of which all our soils 
are made, mingled with the remains of plants and 
animals. Soils differ, therefore, according to the kind 
of rock out of which they have been formed. Sand- 
stone, for example, will give rise to a sandy soil ; 
limestone to a limy or calcareous soil j clay-rocks to a 
clayey soil. 

149. But for this crumbling of the rocks into soil, 
the land would not be covered with verdure as it is. 
Bare sheets of undecaying stone would give no foot- 
ing for the roots of plants. But by the decay of their 
surface, they get covered with fertile soil, all over the 
valleys and plains ; and only where, as in steep banks 
and cliffs, they rise too abruptly to let their crumbled 
remains gather round them, do they stand up naked 
and verdureless. 

150. As the mouldering of the surface of the land is 
always going on, there is a constant formation of soil. 
Indeed, if this were not the case, if after a layer of 
soil had been formed upon the ground, it were to 
remain there unmoved and unrenewed, the plants 
would by degrees take out of it all the earthy materials 
they could, and leave it in a barren or exhausted 
state. But some of it is being slowly carried away 
by rain, fresh particles from mouldering rocks are 
washed over it by the same agent, while the rock or 
sub-soil underneath is all the while decaying into 
soil. The loose stones, too, are continually crumbling 



OF WATER.] PHYSICAL GEOGRAPHY. 6i 

down and making new earth. And thus, day by day, 
the soil is slowly renewed. 

151. Plants, also, help to form and renew the soil. 
They send their roots among the grains and joints of 
the stones, and loosen them. Their decaying fibres 
supply most of the carbonic acid by which these stones 
are attacked, and furnish also most of the organic 
matter in the soil. Even the common worms, which 
you see when you dig up a spadeful of earth, are of 
great service in mixing the soil and bringing what lies 
underneath up to the surface. 

152. When we think about this decay and renewal 
of soil, we see that in reahty the whole surface of the 
land may be looked upon as travelling downward or 
seaward. The particles worn from the sides and 
crests of the high mountains may take hundreds or 
thousands of years on the journey ; they may lie for 
a long time on the slopes ; they may then be swept 
down and form part of the soil of the valleys ; thence 
they may be in after years borne away and laid down 
on the bed or bank of a river ; and thus, after many 
halts by the way, they at last reach the sea. 

153. In order to form some idea of the extent to 
which the surface of the land is cleared of its loose 
soil by rain, you should notice what takes place even 
in this country after every series of heavy showers. 
Each little runnel and brook becomes muddy and 
discoloured from the quantity of soil, that is, decayed 
rock, which is washed into it by the rain from the 
neighbouring slopes. The mud which darkens the 
water is made of the finer particles of the decomposed 
rocks ; the coarser parts are moving along at the bottom 
of the water. When you watch these streamlets at 



62 SCIENCE PRIMERS. [circulation 

their work, and when you remember that what they 
are doing now they have been doing for ages past, you 
will understand how greatly the surface of a country 
may come to be changed by the action of what at 
first seems so insignificant a thing as Rain. 

VI. Brooks and Rivers. Their 'Origin. 

154. We must now go back to an earlier Lesson 
(Art. 107), where the way in which rain is disposed 
of was referred to. You remember that one part of 
the rain sinks under the ground, and you have traced 
its progress there until it comes to the surface again. 
You have now to trace, in a similar way, the other 
portion of the rainfall which flows along the surface in 
brooks and rivers. 

155. You cannot readily meet with a better illus- 
tration of this subject than that which is furnished by 
a gently sloping road during a heavy shower of rain. 
Let us suppose that you know such a road, and that 
just as the rain is beginning you take up your station 
at some part where the road has a well-marked descent. 
At first you notice that each of the large heavy drops 
of rain makes in the dust, or sand, one of the little 
dints or rain-prints already described (Art. 146). As 
the shower gets heavier these rain-prints are effaced, 
and the road soon streams with water. Now mark in 
what manner the water moves. 

156. Looking at the road more narrowly, you re- 
mark that it is full of little roughnesses — at one place 
a long rut, at another a projecting stone, with many 
more inequalities which your eye could not easily 
detect when the road was dry, but which the water at 
once discloses. Every little dimple and projection 



OF WATER.] PHYSICAL GEOGRAPHY. 63 

affects the flow of the water. You see how the 
raindrops gather together into slender streamlets of 
running water which course along the hollows, and 
how the jutting stones and pieces of earth seem to 
turn these streamlets now to one side and now to 
another. 

157. Towards the top of the slope only feeble 
runnels of water are to be seen. But further down 
they become fewer in number, and at the same time 
larger in size. They unite as they descend ; and the 
larger and swifter streamlets at the foot of the descent 
are thus made up of a great many smaller ones from 
the higher parts of the slope. 

158. Now this sloping roadway, with its branching 
rills of rain, coursing down the slope, and uniting into 
larger streams as they advance, shows very well the 
way in which the rain runs off the sloping surface of a 
country or a continent, and we shall return to the 
illustration again. 

159. Why does the water run down the sloping road ? 
why do rivers flow ? and why should they always move 
constantly in the same direction ? They do so for the 
same reason that a stone falls to the ground when it 
drops out of your hand ; because they are under the 
sway of that attraction towards the centre of the eartli, 
to which, as you know, the name of Gravity (Physics 
Primer, Art. 4) is given. Every drop of rain falls to 
the earth because it is drawn downwards by the force 
of this attraction. When it reaches the ground it is 
still, as much as ever, under the same influence ; and it 
flows downwards in the readiest channel it can find. 
Its fall from the clouds to the earth is direct and 
rapid ; its descent from the mountains to the sea, as 



64 SCIENCE PRIMERS. [circulation 

part of a stream, is often long and slow ; but the cause 
of the movement is the same in either case. The 
winding to and fro of streams, the rush of rapids, the 
roar of cataracts, the noiseless flow of the deep sullen 
currents, are all proofs how paramount is the sway of 
the law of gravity over the waters of the globe. 

1 60. Drawn down in this way by the action of 
gravity, all that portion of the rain which does not 
sink into the earth must at once begin to move down- 
wards along the nearest slopes, and continue flowing 
until it can get no further. On the surface of the land 
there are hollows called Lakes, which arrest part of 
the flowing water, just as there are hollows on the 
road which serve to collect some of the rain. But in 
most cases they let the water run out at the lower end 
as fast as it runs in at the upper, and therefore do not 
serve as permanent resting places for the water. The 
streams which escape from lakes go on as before, work- 
ing their way to the sea-shore. So that the course of 
all streams is a downward one ; and the sea is the 
great reservoir into which the water of the land is 
continually pouring. 

161. If the surface of a country were a mere long 
smooth ridge, like the roof of a house, the rain would 
quickly flow down on either side into the sea. But this 
is by no means the general character of the surface of 
the land. Mountains, hills, valleys, gorges, and lakes 
give a most uneven and varied outline. But besides 
these greater inequalities which strike the eye at once, 
even places which seem at first quite level have usually 
some slope or some slight unevennesses ; just as on the 
road you found that there may be many little irregu- 
larities of surface, which you would not notice until 



OF WATER.] PHYSICAL GEOGRAPHY. 65 

the rain found them out. Water is thus a most accu- 
rate measurer of the levels of a country. It will not 
flow up a slope, but always seeks the lowest level it 
can find. 

162. You can see, then, that though the rain should 
fall equally over the whole surface of a country, it can- 
not flow equally over that surface, because the ground 
is uneven, and the rain runs off into the hollows. It 
is this unevenness which makes the rain collect into 
brooks, and these into rivers. 

163. The brooks and rivers of a country are thus the 
natural drains, by which the surplus rainfall, not re- 
quired by the soil or by springs, is led back again into 
the sea. When we consider the great amount of rain, 
and the enormous number of brooks in the higher 
parts of the countr}^, it seems, at first, hardly possible 
for all these streams to reach the sea without over- 
flowing the lower grounds. But this does not take 
place ; for when two streams unite into one, they do 
not require a channel twice as broad as either of their 
single water-courses. On the contrary, such an union 
often gives rise to a stream which is not so broad as 
either of the two from which it flows. But it becomes 
swifter and deeper. In this way thousands of stream- 
lets, as they come together in their descent, are made 
to take up less and. less room, until the surplus waters 
of a whole vast region are borne into the sea by one 
single river-channel. 

164. Let us return to the illustration of the roadway 
in rain. Starting from the foot of the slope, you found 
the streamlets of rain getting smaller and smaller, and 
when you came to the top there were none at all. If, 
however, you were to descend the road on the other 



66 SCJEACE PRIMERS. [circulation 

side of the ridge, you would probably meet with other 
streamlets coursing down-hill in the opposite direction. 
At the summit the rain seems to divide, part flomng 
oft to one side, and part to the other. 

165. In the same way, were you to ascend some 
river from the sea, you would watch it becoming 
narrower as you' traced it inland, and branching more 
and more mto tributaiy streams, and these again 
subdividing into almost endless little brooks. But take 
any of the branches which unite to form the main 
stream, and trace it upward. You come, in the end, 
to the first beginnings of a little brook, and going a 
little further you reach the summit, down the other 
side of which all the streams are flowing to the oppo- 
site -quarter. The line which separates two sets of 
streams in this way is called the Water- shed. In 
England, for example, one series of rivers flows into 
the Atlantic, another into the North Sea. If you trace 
upon a map a line separating all the upper streams of 
the one side from those of the other, that line will 
mark the water-shed of the country. 

166. But there is one important point where the 
illustration of the road in rain quite fails. It is only 
when rain is falling, or immediately after a heavy 
shower, that the rills are seen upon the road. When 
the rain ceases the Avater begins to dr}' up, till in 
a short time the road becomes once more firm and 
dusty. But the brooks and rivers do not cease to flow 
when the rain ceases to fall. In the heat of summer, 
when perhaps there has been no rain for many days 
together, the rivers still roll on, smaller usually than 
they were in winter, but still with ample flow. What 
keeps them full? If you remember what you have 



OF WATER.] PHYSICAL GEOGRAPHY. 67 

» 

already been told about underground-water, you will 
answer that rivers are fed by springs as v/ell 
as by rain. 

167. Though the weather may be rainless, the springs 
continue to give out their supplies of water, and these 
keep the rivers going. But if great drought comes, 
many of the springs, particularly the shallow ones, 
cease to flow, and the rivers fed by them shrink up or 
get dry altogether. This is the case with the rivers of 
this countr}^, which are all, comparatively speaking, 
very small. The great rivers of the globe, such as the 
Mississippi, drain such vast territories, that any mere 
local rain or drought makes no sensible difference in 
their mass of water. 

168. In some parts of the world, however, the rivers 
are larger in summer and autumn than they are in 
winter and spring. The Rhine, for instance, begins 
to rise as the heat of summer increases, and to fall as 
the cold of winter comes on. This happens because 
the river has its source among snowy mountains. 
Snow melts rapidly in summer, and the water which 
streams from it finds its way into the brooks and rivers, 
wliich are thereby greatly swollen. In winter, on the 
other hand, the snow remains unmelted ; the moisture 
which falls from the air upon the mountains is chiefly 
snow; and the cold is such as to freeze the brooks. 
Hence the supplies of water at the sources of these 
rivers are, in winter, greatly diminished, and the rivers 
themselves become proportionately smaller. 

169. Summary. — To sum up what has been stated 
in this and the preceding Lessons regarding the circu- 
lation of water : — From the highest parts of the land 

7 



68 SCIENCE PRIMERS. [circulation 

■ ■ ■ ■ ■ ■ * ■ — - — ■ — ■ 

down to the sea, water is continually travelling down- 
ward. It does not pour over the whole surface, but 
gathers into the hollows, where it forms streams which 
wind to and fro, always seeking a lower level, till at 
last they lose themselves in the sea. From the sea 
vapour is constantly rising into the air, whence it is 
brought back and condensed upon the land as rain 
or snow, which feeds the streams that flow downward 
into the sea. This circulation of water goes on with- 
out ceasing. 

VII. Brooks and Rivers. Their work. 

170. In the first lesson of this little book you were 
asked to watch, the doings of a river. Let us now 
again return to the same scene, but before the storm 
which was then described. The river is not yet swollen 
with the sudden and heavy rain. It flows gently over 
its pebbly channel, not covering the whole of it, per- 
haps, but leaving banks of gravel and pools of water 
between which the clear current, much diminished by 
drought, winds its way. The river seems to be doing 
nothing else than lazily carrying the surplus water of 
the land towards the sea. You might be surprised to 
be told that it has any work to do, and even now is 
doing it. 

171. But consider whence the water of the river 
comes. We have found that it is largely derived from 
springs, and that all spring-water contains more or less 
mineral materials dissolved out of the brooks. Every 
river, therefore, is carrying not merely water, but large 
quantities of mineral matter into the sea. It has been 
calculated, for instance, that the Rhine in one year 
carries into the North Sea lime enough to make three 



OF WATER.] PHYSICAL GEOGRAPHY. 69 

hundred and thirty-two thousand millions of oyster 
shells. This chemically-dissolved material is not 
visible to the eye, and in no way affects the colour of 
the water. At all times of the year, as long as the 
water flows, this invisible transport of some of the 
materials of rocks must be going on. 

172. But let us now again watch the same river in 
flood. The water is no longer clear, but dull and dirty. 
Y5u ascertained that this discoloration arises from 
mud and sand suspended in the water. You may stand 
for hours and watch the swollen, turbid torrent rolling 
down its channel. During that time many tons of 
gravel, sand, and mud must be swept past you. 
You see that over and above the mineral matter in 
chemical solution, the river is hurrying seaward with 
vast quantities of other and visible materials. And 
thus it is clear that at least one great part of the work 
of rivers must be to transport the mouldered parts of 
the land which are carried into them by springs or 
by rain. 

173. But the rivers, too, help in the general destruc- 
tion of the surface of the land. Of this you may readily 
be assured, by looking at the sides or bed of a stream 
when the water is low. Where the stream flows over 
hard rock, you find the rock all smoothed and ground 
away ; and the stones lying in the water-course are 
all more or less rounded and smoothed. When these 
stones were originally broken by frosts or otherwise, 
from crags and clitfs, they were sharp-edged, as you 
can prove by looking at the heaps of blocks lying at 
the foot of any precipice, or steep bank of rock. But 
when they fell, or were washed into the river, they 
began to get rolled and rubbed, until their sharp edges 



70 



SCIENCE PRIMERS. [circulation 



were ground away, and they came to wear the smooth 
rounded forms which we see in the ordinary gravel. 

174. While the stones are ground down, they, at 
the same time, grind down the rocks which form the 




Fig. 10. — Potholes excavated by a Stream in the Rocks of its Bed. 

sides and bottom of the river-channel over which they 
are driven. You can even see in some of the eddies 
of the stream how the stones are kept moving round 



OF WATER.] PHYSICAL GEOGRAPHY. 71 

until they actually excavate deep round cavities, called 
pot-holes, in the solid rock. When the water is low, 
as during the droughts of summer, some of these cavi- 
ties are laid bare, and you may then observe how well 
they have been polished. Their general appearance 
is shown in Fig. 10. 

175. Now, it is clear that two results must follow 
from this ceaseless wear and tear of rocks and stones 
in the channel of a stream. In the first place, a great 
deal of mud and sand must be produced ; and, in the 
second place, the bed of the river must be ground 
down so as to become deeper and wider. The sand 
and mud are added to the other similar materials 
washed into the streams by rain from the mouldering 
surface of the land. By the deepening and widening 
of the water-courses, such picturesque features as 
gorges and ravines are excavated out of the solid 
rock. 

176. You have now seen why the rivers are muddy. 
Let us inquire what becomes of all the mud, sand, 
gravel, and blocks of stone which they are continually 
transporting. 

177. Look, again, at the channel of a river in sum- 
mer. You see it covered with sheets of gravel in one 
place, beds of sand in another, while here and there 
a piece of hard rock sticks up through these different 
kinds of river-stuff. Note some portion of the loose 
materials, and you find it to be continually shifting. A 
patch of gravel or sand may remain for a time, but 
the little stones and grains of which it is made up 
are always changing as the water covers and moves 
them. In fact, the loose materials over which the river 
flows are somewhat like tlie river itself You come 



72 SCIENCE PRIMERS. [circulation 

back to its banks after many years, and you find the 
river there still, with the same ripples, and eddies, and 
gentle murmuring sound. But though the river has 
been there constantly all the time, its water has been 
changing every minute, as you can watch it changing 
still. So, although the channel is always more or less 
covered with loose materials, these are not always the 
same. They are perpetually being pushed onward, 
and others, from higher up the stream, come behind 
to take their place. 

178. It is not in the bottoms of the rivers, then, that 
the material worn away from the surface of the land can 
find any lasting rest. And yet the rivers do get rid of 
a good deal of this material as they roll along. You 
have, perhaps, noticed that a river is often bordered 
with a strip of flat plain, the surface of which is only 
a few feet above the level of the water. Most of our 
rivers have such margins, and, indeed, seem each to 
wind to and fro through a long, level, meadow-like 
plain. Now this plain is really made up from the finer 
particles of the decomposed rocks which the river has 
carried along. During floods, the river, swollen and 
muddy, rises above its banks, and spreads over the low 
ground on either side. Whenever this takes place, the 
overflowing water moves more slowly over the flats ; 
and, as its current is thus checked, it cannot hold so 
much mud and sand, but allows some of these ^naterials 
to settle down to the bottom. In this way the over- 
flowed tracts get a coating of soil laid over them 
by the river, and when the waters retire this coating 
adds a little to the height of the plain. The same 
thing takes place year after year, until by degrees the 
plain gets so far raised that the river, which all this 



OF WATER.] PHYSICAL GEOGRAPHY. 73 



while is also busy deepening its channel, cannot over- 
flow it even at the highest floods. In course of time 
the river, as it winds from side to side, cuts away 
slices of the plain and forms a newer one at a lower 
level. And thus a series of terraces is gradually made, 
rising step by step above the river. 



Fig. II. — Section of the successive terraces (i,.2, 3! of sand, earth, and 
gravel formed by a River along a valley (s— s). 

179. Still the laying down of its sand and mud by 
a river to form one or more such river-terraces is, 
after all, only a temporary disposal of these materials. 
They are still liable to be carried away, and in truth 
they are carried off continually as the river eats away 
its banks. 

180. When the current of a river is checked as it 
enters the sea or a lake, the feebler flow of the water 
allows the sand and mud to sink to the bottom. By 
degrees some portions of the bottom come in this 
way to be filled up to the surface of the river, and 
wide flat marshy spaces are formed on either side of 
the main stream. During floods these spaces are 
overflowed with muddy water, in the same way as in 
the case of the valley plains just described, and a 
coating of mud or sand is laid down on them until 
they slowlv rise above the ordinary level of the river, 
which winds about among them in endless branching 
streams. Vegetation springs up on these flat swampy 
lands ; animals, too, find food and shelter there ; and 



74 



SCIENCE PRIMERS. [circulation 



thus a new territory is made by the work of the 
river. 

1 8 1. These flat river-formed tracis are called Deltas, 
because the one which was best known to the ancients, 
that of the Nile, had the shape of the Greek letter 
A {delta). This is the general form which is taken 




Fig. 12. — Delta of the Mississippi. 



by accumulations at the mouths of rivers ; the flat 
delta gets narrow towards the inland, and broader 
towards the sea. Some of them are of enormous 
size ; the delta of the Mississippi, for example. 

182. Each delta, then, is made of materials worn 
from the surface of the land, and brought down by the 



OF WATER.] PHYSICAL GEOGRAPHY. ' 75 

river. And yet vast though some of these deltas are, 
they do not show all the materials which have been 
so worn away. A great deal is carried far out and 
deposited on the sea-bottom ; for the sea is the great 
basin into which the spoils of the land are continually 
borne. 

VIII. Snow-fields and Glaciers. 

183. Having now followed the course taken by the 
water which falls on the land as rain, we come to that 
taken by snow (Art. 92). 

184. On the tops of some of the highest mountains 
in Britain snow lies for great part of the year. On 
some of them, indeed, there are shady clefts wherein 
you may meet with deep snow-wreaths even in the 
heat of summer. It is only in such cool and sheltered 
spots, however, that the snow remains unmelted. 

185. But in other parts of Europe, where the moun- 
tains are more lofty, the peaks and higher shoulders of 
the hills gleam white all the year with unmelted snow. 
Hardly anything in the world will impress you so 
much as the silence and grandeur of these high snowy 
regions. Seen from the valleys, the mountains look so 
vast and distant, so white and pure, yet -catching up 
so wonderfully all the colours which glow in the sky 
at morn or even, that they seem to you at first rather 
parts of the heaven above than of the solid earth on 
which we live. But it is when you climb up fairly 
into their midst thnt their wonderful stateliness comes 
full before you. Peaks and pinnacles of the most 
dazzling whiteness glisten against the dark blue of the 
sky, streaked here and there with lines of purple 
shadow, or with knobs of the dark rock projecting 



76 SCIENCE PRIMERS. [circulation 

through the white mantle which throws far and wide 
its heavy folds over ridge and slope, and sends long 
tongues of blue ice down to the meadows and vine- 
yards of the valleys. There is a deep silence over 
this high frozen country. Now and then a gust of 
wind brings up from the far distance the sound of 
some remote waterfall or the dash of a mountain 
torrent. At times, too, there comes a harsh roar as 
of thunder, when some mass of ice or snow, loosened 
from the rest, shoots down the precipices. But these 
noises only make the silence the deeper when they 
have passed away. 

1 86. Let us see why it is that perpetual snow should 
occur in such regions, and what part this snow plays 
in the general machinery of the world. 

187. You have learnt (Art. 96) that the higher parts 
of the atmosphere are extremely cold. You know also 
that in the far north and the far south, around those 
two opposite parts of the earth's surface called the 
Poles, the chuiate is extremely cold— so cold as to give 
rise to dreary expanses of ice and snow, where sea and 
land are frozen, and where the heat of summer is not 
enough to thaw all the ice and drive away all the 
snow. Between these two polar tracts of cold, wher- 
ever mountains are lofty enough to get into the high 
parts of the atmosphere where the temperature is 
usually below the freezing-point, the vapour condensed 
from the air falls upon them, not as rain, but as snow. 
Their heads and upper heights are thus covered with 
perpetual snow. In such high mountainous regions 
the heat of the summer always melts the snow from, 
the lower hills, though it leaves the higher parts still 
covered. From year to year it is noticed that there is 



OF WATER.] PHYSICAL GEOGRAPHY. 77 

a line or limit below which the ground gets freed of 
its snow, and above which the snow remains. This 
limit is called the snow- line, or the limit of 
perpetual snow. Its height varies in different 
parts of the world. It is highest in the warmer 
regions on either side of the equator, where it reaches 
to 15,000 feet above the sea. In the cold polar 
tracts, on the other hand,' it approaches the sea-level. 
In other words, while in the polar tracts the climate 
is so cold that perpetual snow is found even close 
to the sea-level, the equatorial regions are so warm 
that you must climb many thousand feet before you 
can reach the cold layers of the air where snow can 
remain all the year. 

188, You have no doubt watched a snow-storm. 
You have seen how at first a few flakes begin to show 
themselves drifting through the air ; how they get 
more in number and larger in size, until the ground 
begins to grow white ; and how, as hours go on, the 
whole country becomes buried under a white pall, 
perhaps six inches or more in thickness. You see 
one striking difference between rain and snow. If 
rain had been falling for the same length of time, 
the roads and fields would still have been visible, for 
each drop of rain, instead of remaining where it fell, 
would either have sunk into the soil, or have flowed 
off into the nearest brook. But each snowflake, on 
the contrary, lies where it falls, unless it happens to 
be caught up and driven on by the wind to some 
other spot where it can finally rest. Rain disappears 
from the ground as soon as it can ; snow stays still 
as long as it can. 

189. You will see at once that this marked differ- 



78 SCIENCE PRIMERS. [circulation 



ence of behaviour must give rise to some equally 
strong differences in the further procedure of these 
two kinds of moisture. You have followed the pro- 
gress of the rain; now let us try to find out what 
becomes of the snow. 

190. In such a country as ours, where there is 
no perpetual snow, you can without much difficulty 
answer this question. Each fall of snow in winter-time 
remains on the ground as long as the air is not warm 
enough to melt it. Evaporation, indeed, goes on from 
the surface of snow and ice, as well as from water; so 
that a layer of snow would in the end disappear, by 
being absorbed into the air as vapour, even though none 
of it had previously been melted into running water. 
But it is by what we call a thaw that our snow is 
chiefly dissipated ; that is, a rise in the temperature, 
and a consequent melting of the snow. When the 
snow melts, it sinks into the soil and flows off into 
brooks in the same way as rain. Its after course 
needs not to be followed, for it is the same as that 
of rain. You will only bear in mind that if a 
heavy fall of snow should be. quickly thawed, then 
a large quantity of water will be let loose over the 
country, and the brooks and rivers will rise rapidly 
in flood. Great destruction may thus be caused by 
the sudden rise of rivers and the overflowing of their 
banks. 

191. In the regions of perpetual snow the heat of 
summer cannot melt all the snow which falls there in 
the year. What other way of escape, then, can the 
frozen moisture find ? That it must have some means 
of taking itself off the mountains is clear enough ; for 
if it had not, and if it were to accumulate there from 



OF WATER.] PHYSICAL GEOGRAPHY. 79 

year to year and from century to centur}-, then the 
mountains would grow into vast masses of snow, reach- 
ing far into the sky, and spreading out on all sides, 
so as to bury by degrees the low lands around. But 
nothing of this kind takes place. These solemn 
snowy heights wear the same unchanged look from 
generation to generation. There is no bur}dng of 
their well-known features under a constantly increas- 
ing depth of snow. 

192. You will remember that the surplus rainfall 
flows off by means of rivers. Now the surplus 
snow-fall above the snow-line has a similar kind of 
drainage. It flows off by means of what are called 
Glaciers. 

193. When a considerable depth of snow has accu- 
mulated, the pressure upon the lower layers from 
what lies above them squeezes them into a firm 
mass. The surface of the ground is usually sloped 
in some direction, seldom quite flat. And among 
the high mountains the slopes are often, as you 
know, very steep. When snow gathers deeply on 
sloping ground, there comes a time when the force 
of gravity overcomes the tendency of the pressed 
snow to remain where it is, and then the snow- 
begins to slide slowly down the slope. From one 
slope it passes on downwards to the next, joined 
continually by other sliding masses from neighbour- 
ing slopes until they all unite into one long tongue 
which creeps slowly down some valley to a point 
where it melts. This tongue from the snow-fields is 
the glacier. It really drains these snow-fields of their 
excess of snow as much as a river drains a district of 
its excess of water. 

8 



8o SCIENCE PRIMERS. [circulation 

194. But the glacier which comes out of the snow- 
fields is itself made not of snow, but of ice. The 
snow, as it slides downward, is pressed together into 
ice. You have learned that each snowflake is made 
of little crystals of ice. A mass of snow is thus only 
a mass of minute crystals of ice with air between. 
Hence when the snow gets pressed together, the air 
is squeezed out, and the separated crystals of ice 
freeze together into a solid mass. You know that 
you can make a snowball very hard by squeezing it 
firmly between the hands. The more tightly you 
press it the harder it gets. You are doing to it just 
what happens when a glacier is formed out of the 
eternal snows. You are pressing out the air, and 
allowing the little particles of ice to freeze to each 
other and form a compact piece of ice. But you 
cannot squeeze nearly all the air out, consequently 
the ball, even after all your efforts, is still white from 
the imprisoned air. Among the snowfields, however, 
the pressure is immensely greater than yours ; the air 
is more and more pressed out, and at last the snow 
becomes clear transparent ice. 

195. A glacier, then, is a river, not of water, but of 
ice, coming down from the snow-fields. It descends 
sometimes a long way below the snow-line, creeping 
down very slowly along the valley which it covers 
from side to side. Its surface all the tim.e is melting 
during the day in summer, and streams of clear 
water are gushing along the ice, though, when night 
comes, these streams freeze. At last it reaches some 
point in the valley beyond which it cannot go, for 
the warmth of the air there is melting the ice as fast 
as it advances. So the glacier ends, and from its 



OF WATER.] PHYSICAL GEOGRAPHY. 



8i 



melting extremity streams of muddy water unite into 
a foaming river, which bears down the drainage of 
the snow-fields above. 

196. In the accompanying woodcut (fig. 13) some of 
the chief characters of a glacier are shown. In the dis- 
tance rise the snowy heights, among which the snow- 




FiG. 13. — View of a Glacier, with its Moraines, Perched Blocks of rock, ice- 
worn Bosses of rock and escaping River. 



fields lie. From either side the snow is drained oft" 
into the main valley, where it forms the glacier, which 
winds with all the windino:s of the vallev till it ends 
abruptly, as you see, and a river rushes out from the 
melting end of the ice. 

197. A river wears down the sides and bottom of 
its channel, and thus digs out a bed for itself in even 



82 SCIENCE PRIMERS. [circulation 

the hardest rock, as well as in the softest soil 
(Art. 173). It sweeps down, too, a vast quantity of 
mud, sand, and stones from the land to the sea 
(Art. 172). A glacier performs the same kind of 
work, but in a very diiferent way. 

198. When stones fall into a river they sink to the 
bottom, and are pushed along there by the current. 
When mud enters a river it remains suspended in the 
water, and is thus carried along. But the ice of a 
glacier is a solid substance. Stones and mud which 
fall upon its surface remain there, and are borne 
onward with the whole mass of the moving glacier. 
They form long lines of rubbish upon the glacier, as 
shown in fig. 13, and are called moraines. Still 
the ice often gets broken up into deep cracks, opening 
into yawning clefts or crevasses, which sometimes 
receive a good deal of the earth and stones let loose 
by frost or otherwise from the sides of the valley. In 
this way loose materials fall to the bottom of the ice, 
and reach the sohd floor of the vallev down which the 
ice is moving; while at the same time similar rubbish 
tumbles between the edge of the glacier and the side 
of the valley. 

199. The stones and grains ofsand which get jammed 
between the ice and the rock over w^hich it is moving 
are made to score and scratch this rock. They form 
a kind of rough polishing powder, whereby the glacier 
is continually grinding down the bottom and sides of 
its channel. If you creep in below the ice, or catch a 
sight of some part of the side from which the ice has 
retired a little, you will -find the surface of the rock all 
rubbed away and covered with long scratches made 
by the sharp points of the stones and sand. Some of 



OF WATER.] PHYSICAL GEOGRAPHY. 



83 



the rounded ice-worn bosses of rock are shown in the 
fore-ground of the diagram (fig. 13). 

200. You will now see the reason why the river, 
which escapes from the end of a glacier, is always 
muddy. The bottom of the glacier is stuck all over 
w^th stones, which are scraping and wearing down the 
rock underneath. A great deal of fine mud is thus 
produced, which, carried along by streams of water 
flowing in channels under the glacier, emerges at the 
far end in the discoloured torrents which there sweep 
from under the ice. 




Fig. 14. — Loose stone polished and scratched under glacier-ice. 



201. A glacier is not only busy grinding out a bed 
for itself through the mountains ; it bears on its back 
down the valley enormous quantities of fallen rock, 
earth, and stones, which have tumbled from the cliffs 
on either side. In this way blocks of rock as big as 
a house may be carried for many miles, and dropped 
where the ice melts. In the following figure (fig. 15) you 
have a drawing of one of these huge masses of stone. 
Thousands of tons of loose stones and mud are every 
year moved on the ice from the far snowy moun- 



84 



SCIENCE PRIMERS. [circulation 



tains a.way down into the valleys to which the glaciers 
reach. 

202. The largest glaciers in the world are those of the 
polar regions. Norch Greenland, in truth, lies buried 
under one great glacier, which pushes long tongues of 
ice down the valleys and away out to sea. When a 
glacier advances into the sea, portions of it break off 
and float away as icebergs (fig. 16). So enormous are 




Fig. is.-Erratic block, brought from the Alps by an ancient Glacier, and 
dropped upon the Jura Mountains. 

the glaciers in these cold tracts that the icebergs de- 
rived from them often rise several hundred feet above 
the waves which beat against their sides. And yet, in 
all such cases, about seven times more of the ice is 
immersed under water than the portion, large as it is, 
which appears above. You can realize how this happens 
if you take a piece of ice, put it in a tumbler of water, 
and watch how much of it rises out of the water. 



OF WATER.] PHYSICAL GEOGRAPHY. 



85 



Sunk deep in the sea, therefore, the icebergs float 
to and fro until they melt, sometimes many hundreds 
of miles away from the glaciers which supplied them. 
203. You will come to learn afterwards that, once 
upon a time, there were glaciers in Britain. You will 
be able with your own eyes to see rocks which have 
been ground down and scratched by the ice, and big 
blocks of rock and piles of loose stones which the ice 




Fig. 16. — Iceberg at Pea. 

carried upon its surface. In Wales, and Cumberland, 
in many parts of Scotland, and also in Ireland, these 
and many other traces of the ice are to be found. So 
that, in learning about glaciers, you are not merely 
learning what takes place in other and distant lands, 
you are gaining knowledge which you will be able 
by and by to make good use of, even in your own 
country. 



86 SCIENCE PRIMERS. [the 



THE SEA. 

I. Grouping of Sea and Land. 

204. Since we live on land, and are familiar with 
the various shapes which the surface of the land 
assumes, — plains, valleys, hills, mountains, and so on, 
— we are apt to think that the land is the main part 
of the globe. Many of us who live in the inland 
parts of the country have never been off the land, nor 
seen any larger sheet of water than a river or a lake, 
or perhaps a large reservoir. And yet, if you were to 
travel onward in any direction in Great Britain, you 
would at last come to the edge of the land, and find a 
vast expanse of water before you. If you took your 
place in a ship, you could sail on that water com- 
pletely round this country, and you would prove in 
so doing that Britain is an island. 

205. Suppose that instead of sailing round Britain, 
which you could easily do in a few weeks, you were 
to steer straight westward. You would have to travel 
over the water for more than two thousand miles 
before you reached any land again. Or, if you di- 
rected your ship in a more southerly course, you might 
sail on without seeing any land for months together, 
until you came in sight of the ice-cliffs that border 
the land round the South Pole. You would learn in 
this way what an enormous extent of the surface of 
the earth is occupied by water. 

206. It has been ascertained that in reality the 
water covers about three times more of the earth's 
surface than the land does. ^Ve could not tell that 
merely by what we can see from any part of this 
country, or indeed of any country. It is because 



SEA.] PHYSICAL GEOGRAPHY. 87 

men have sailed round the world, and have crossed 
it in many directions that the proportion of land and 
water has come to be known. 

207. Take a school-globe, and turn it slowly round 
on its axis. You see at a glance how much larger the 
surface of water is than the surface of land. But you 
may notice several other interesting things about the 
distribution of land and water. 

208. In the first place, you will find that the water 
is all connected together into one great mass, which 
we call the sea. The land, on the other hand, is 
much broken up by the way the sea runs into it ; and 
some parts are cut off from the main mass of land, 
so as to form islands in the sea. Britain is one of the 
pieces of land so cut off. 

209. In the second place, you cannot fail to notice 
how much more land lies on the north than on the 
south side of the equator. If you turn the globe so 
that your eye shall look straight down on the site of 
London, you will find that most of the land on the 
globe comes into sight ; whereas, if you turn the 
globe exactly round, and look straight down on the 
area of New Zealand, you will see most of the sea. 
London thus stands about the centre of the land- 
hemisphere, midway among the countries of the earth. 
And no doubt this central position has not been 
without its influence in fostering the progress of 
British commerce. 

210. In the third place, you will notice that by the 
way in which the masses of land are placed, parts of 
the sea are to some extent separated from each other. 
These masses of land are called continents, and 
the wide sheets of water between are termed oceans. 



88 SCIENCE PRIMERS. [the 

Picture to yourselves that the surface of the solid 
part of the earth is uneven, some portions rising into 
broad swellings and ridges, others sinking into wide 
hollows and basins. Now, into these hollows the sea 
has been gathered, and only those upstanding parts 
which rise above the level of the sea form the land. 

211. In the foregoing parts of this little book men- 
tion has often been made of the Sea. You have been 
told that the moisture of the air comes in great part 
from the sea ; that the rivers of the land are continually 
flowing into the same reservoir of water, which is like- 
wise the great basin into which all the soil which is 
worn from the surface of the land is carried. We 
must now look a little more closelv at some of the 
more important features of the sea. 

II. Whv the Sea is Salt. 

2 12. When you come to examine the water of the 
sea, you find that it differs from the water with which 
you are famiHar on the land, inasmuch as it is salt. 
It contains something which you do not notice in 
ordinary spring or river water. If you take a drop of 
clear spring-water, and allow it to evaporate from a 
piece of glass, you will find no trace left behind. The 
water of springs, as you have already leanit (Art. 117), 
always contains some mineral substances dissolved in 
it, and these not being capable of rising in vapour 
are left behind when the water evaporates. But the 
quantity of them in a single drop of water is so 
minute that, when the drop dries up, it leaves no per- 
ceptible speck or film. Take, however, a drop of sea- 
water, and allow it to evaporate. You find a little 
white point or film left behind, and on placing that 



SEA.] PHYSICAL GEOGRAPHY. 89 

film under a microscope you see it to consist of delicate 
crystals of common or sea salt. It would not matter 
from what ocean you took the drop of water, it would 
still show the crystals of salt on being evaporated. 

213. There are some other things besides common 
salt in sea-water. But the salt is the most abundant, 
and we need not trouble about the rest at present. 
Now^, where did all this mineral matter in the sea 
come from? The salt of the sea is all derived 
from the waste of the rocks. 

214. It has already been pointed out (Arts. 125, 
132) how, both underground and on the surface of 
the land, water is always dissolving out of the rocks 
various mineral substances, of which salt is one. 
Hence the water of springs and rivers contains salt, 
and this is borne away into the sea. So that all 
over the world there must be a vast quantity of salt 
carried into the ocean every year. 

215. The sea gives off again by evaporation as much 
water as it receives from rain and from the rivers of 
the land. But the salt carried into it remains behind. 
If you take some salt water and evaporate it, the pure 
water disappears, and the salt is left. So it is with 
the sea. Streams are every day carrying fresh supplies 
of salt into the sea. Every day, too, millions of tons 
of water are passing from the ocean into vapour in 
the atmosphere. The waters of the sea must con- 
sequently be getting Salter by degrees. The process, 
however, is an extremely slow one. 

216. Although sea- water has probably been gradually 
growins: in saltness ever since rivers first flowed into 
the great sea, it is even now by no means as salt as 
it might be. In the Atlantic Ocean, for example, the 



go SCIENCE PRIMERS. [the 



total quantity of the different salts amounts only to 
about three and a half parts in every hundred parts 
of water. But in the Dead Sea, which is extremely . 
salt, the proportion is as much as twenty-four parts in 
the hundred of water. 

III. The Motions of the Sea. 

217. Standing by the shore of any part of Britain, 
and watching for a little the surface of the sea, you 
noiice how restless it is. Even on the calmest summer 
day, a slight ripple or a gentle heaving motion will be 
seen ; at other times little wavelets curl towards the 
land, and break in long lines upon the beach ; but 
now and then, when storms arise, you may watch how 
the water has been worked up into huge billows which, 
crested with spray, come in, tossing and foaming, to 
burst upon the shores. 

218. Again, if you watch a little longer, you will find 
that whether the sea is calm or rough, it does not 
remain always at the same limit upon the beach. At 
one part of the day the edge of the water reaches to 
the upper part of the sloping beach ; some six hours 
afterwards it has retired to the lower part. You may 
watch it falling and rising, day after day, and year 
after year, with so much regularity that its motion can 
be predicted long beforehand. This ebb and flow of 
the sea forms what are called tides. 

219. If you cork up an empty bottle and throw it 
into the sea, it will of course float. But it will not 
remain long where it fell. It will begin to move 
away, and may travel for a long distance until thrown 
upon some shore again. Bottles cast upon mid-ocean 
have been known to be carried in this way for many 



SEA.] PHYSICAL GEOGRAPHY. 91 



hundreds of miles. This surface -drift of the sea- 
water corresponds generally 'with the direction in 
which the prevalent winds blow. 

220. But it is not merely the surface-water which 
moves. You have learnt a little about icebergs 
(Art. 202); and one fact about them which you must 
remember is that, large as they may seem, there is 
about seven times more of their mass below water than 
above it. Now, it sometimes happens that an iceberg 
is seen sailing on, even right in the face of a strong 
wind. This shows that it is moving, not with the 
wind, but with a strong under-current in the sea. In 
short, the sea is found to be traversed by many 
currents, some flowing from cold to warm regions, 
and others from warm to cold. 

221. Here, then, are four facts about the sea: — 
I St, it has a restless surface, disturbed by ripples and 
waves ; 2ndly, it is constantly heaving with the ebb 
and flow of the tides ; 3rdly, its surface-waters drift 
with the wind ; and 4thly, it possesses currents like 
the atmosphere. 

222. For the present it will be enough if we learn 
something regarding the first of these facts — the 
waves of the sea. 

223. Here again you may profitably illustrate by 
familiar objects what goes on upon so vast a scale in 
nature. Take a basin, or a long trough of water, and 
blow upon the water at one edge. You throw its 
surface into ripples, which, as you will observe, start 
from the place where your breath first hits the water 
and roll onward until they break in little wavelets 
upon the opposite margin of the basin. 

224. What you do in a small way is the same action 

9 



92 SCIENCE PRIMERS. [the 

by which the waves of j;he sea are formed. All these 
disturbances of the smoothness of the sea are due to 
disturbances of the air. Wind acts upon the water of 
the sea as your breath does on that of the basin. 
Striking the surface, it throws the water into ripples 
or undulations, and in continuing to blow along the 
surface it gives these additional force, until driven 
on by a furious gale they grow into huge billows. 

225. When waves roll in on the land, they break 
one after another upon the shore, as your ripples 
break upon the side of the basin. And they continue 
to roll in after the wind has fallen, in the same v^^ay 
that the ripples in the basin will go on curling for a 
little after you have ceased to blow. The surface of 
the sea, like that of water generally, is very sensitive. 
If it is thrown into undulations, it does not become 
motionless the moment the cause of disturbance has 
passed away, but continues moving in the same way, 
but in a gradually lessening degree, until it comes 
to rest. 

226. The restlessness of the surface of the sea 
becomes in this way a reflection of the restlessness of 
the air. It is the constant moving to and fro of cur- 
rents of air, either gentle or violent, which roughens 
the sea with waves. When the air for a time is calm 
above, the sea sleeps peacefully below \ when the sky 
darkens, and a tempest bursts forth, the sea is lashed 
into waves, which roll in and break with enormous 
force upon the land. 

227. You have heard, perhaps you have even seen, 
something of the destruction which is worked by the 
waves of the sea. Every year piers and sea-walls are 
broken down, pieces of the coast are washed away. 



SEA.] 



PHYSICAL GEOGRAPHY. 



93 



and the shores are strewn with the wret.k of ships. 
So that, besides all the waste which the surface of the 
land undergoes from rain, and frost, and streams, 
there is another form of destruction going on along 
the coast-line. 

228. On rocky shores the different stages in the 
eating away of the land by the sea can sometimes 
be strikingly seen. Above the beach perhaps rises a 
.cliff, sorely battered about its base by the ceaseless 





.-\ 









Fig. 17. — Coast-line worn by the S«a. 



grinding of the waves. Here and there a cavern has 
been drilled in the solid wall, or a tunnel has been 
driven through some ])rojecting headland. Not f^ir off 
we may note a tall buttress of rock, once a part of the 
main cliff, but now separated from it by the falhng in 
and removal of the connecting archway. And then, 
further off from the cliff, isolated, half-tide rocks 
rise to show wliere still older detached buttresses 



94 SCIENCE PRIMERS. [the 

Stood ; while away out in the sea the dash of breakers 
marks the site of some sunken reef, in which we see 
the reUcs of a still more ancient coast-line. On su6h 
a shore the whole process whereby the sea eats into 
the land seems to be laid open to our eyes. 

229. On some parts of the coast-line of the east of 
England, where the rock is easily worn away, the 
sea advances on the land at a rate of two or three 
feet every year. Towns and villages which existed a 
i^"^ centuries ago have one by one disappeared, and 
their sites are now a long way out under the restless 
waters of the North Sea. On the west coast of Ire- 
land and Scotland, however, where the rocks are 
usually hard and resisting, the rate of waste has been 
comparatively small. 

230. It would be worth your while the first time 
you happen to be at the coast to ascertain what 
means the sea takes to waste the land. This you can 
easily do by watching what happens on a rocky beach. 
Get to some sandy or gravelly part of the beach, 
over which the waves are breaking, and keep your 
eye on the water when it runs back after a wave 
has burst. You see all the grains of gravel and sand 
hurrying down the slope with the water; and if the 
gravel happens to be coarse, it makes a harsh grating 
noise as its stones rub agairst each other — a noise 
sometimes loud enough to be heard miles away. As 
the next wave comes curling along, you will mark 
that the sand and gravel, after slackening their 
downward pace, are caught up by the bottom of the 
advancing wave and dragged up the beach again, 
only to be hurried down once more as the water 
retires to allow another wave to do the same work. 



SE.\.] PHYSICAL GEOGRAPHY. 95 

231. By this continual up and do\Mi movement of 
the water, the sand and stones on the beach are 
kept grinding against each other, as in a mill. 
Consequently they are worn away. The stones 
become smaller, until they pass into mere sand, 
and the sand, growing finer, is swept away out 
to sea and laid down at the bottom. 

232. But not only the loose materials on the 
shore suffer in this way an incessant wear and tear, 
the solid rocks underneath, wherever they come to 
the surface, are ground down in the same process. 
When the Avaves dash against a cliff they hurl the 
loose stones forward, and batter the rocks ^^'ith them. 
Here and there in some softer part, as in some crevice 
of the cliff, these stones gather together, and when the 
sea runs high they are kept whirling and grinding at 
the base of the cliff till, in the end, a cave is actually 
bored by the sea in the solid rock, very much in 
the same way as, you remember (Art. 174), we saw 
that holes are bored by a river in the bed of its 
channel. The stones of course are ground to sand 
in the process, but their place is supplied by others 
swept up by the waves. If you enter one of these 
sea-caves when the water is low, you will see how 
smoothed and polished its sides and roof are, and 
how well rounded and worn are the stones lying 
on its floor. 

IV. The Bottom of the Sea. 

233. So far as we know, the bottom of the sea is 
ver)' much like the surface of the land. It has heights 
and hollows, lines of valleys and ranges of hills. We 
cannot see down to the bottom where the water is 



96 SCIENCE PRIMERS. [the 

very deep, but we can let down a long line with a 
weight tied to the end of it, and find out both how 
deep the water is, and what is the nature of the 
bottom, whether rock or gravel, sand, mud, or shells. 
This measuring of the depths of the water is called 
Sounding, and the weight at the end oi the line 
goes by the name of the Sounding-lead. 

234. Soundings have been made over many parts 
of the sea, and something is now known about its 
bottom, though much still remains to be discovered. 
The Atlantic Ocean is the best known. In sounding 
it, before laying down the telegraphic cable which 
stretches across under the sea from this country to 
America, a depth of 14,500 feet, or two miles and 
three-quarters, was reached. But between the Azores 
and the Bermudas a sounding has been obtained of 
seven miles and a half. If you could lift up the 
Himalaya mountains, which are the highest on the 
globe, reaching a height of 29,000 feet above the sea, 
and set them down in the deepest part of the Atlantic, 
they would not only sink out of sight, but their tops 
would actually be about two miles below the surface. 

235. A great part of the wide sea must be one or 
two miles deep. But it is not all so deep as that, for 
even in mid-ocean some parts of its bottom rise up to 
the surface and form islands. As a rule it deepens in 
the tracts furthest from land, and shallows towards 
the land. Hence those parts of the sea which run 
in among islands and promontories are, for the most 
part, comparatively shallow. To the west of the 
island of Great Britain, stretches the wide Atlantic 
Ocean ; to the east lies the much smaller North Sea ; 
the former soon getting very deep as we sail west- 



SEA.] PHYSICAL GEOGRAPHY. 97 

wards across it, the latter never deepening much even 
over its middle parts, which are nowhere so much 
as 400 feet below the surface. You may get some 
notion of the shallowness of the sea between this 
country and France, when you are told that if you 
could lift St. Paul's cathedral from London, and set 
it down in the middle of the Strait of Dover, more 
than a half of the building would be out of the water. 

236. You may readily enough understand how it is 
that soundings are made, though you can see how 
difficult it must be to work- a sounding-line several 
miles long. Yet men are able not only to measure 
the depth of the water, but by means of the instru- 
ment called a dredge, to bring up bucketfuls of 
whatever may be lying on the sea-floor, from even the 
deepest parts of the ocean. In this way during the 
last few years a great deal of additional knowledge 
has been gathered as to the nature of the sea-floor, 
and the kind of plants and animals which live there. 
We now know that even in some of the deepest 
places which have yet been dredged there is plenty 
of animal life, such as shells, corals, star-fishes, and 
still more humble creatures. 

237. In earlier parts of this book we have traced 
some of the changes which from day to day take 
place upon the surface of the land. Let us now try 
to watch some of those which go on upon the floor of 
the sea. We cannot, indeed, examine the sea-bottom 
with anything like the same minuteness as the surface 
of the land. Yet a great deal may be learnt regard- 
ing it. 

22,'^. If you put together some of the acts with 
which we have been dealing in the foregoing 



98 SCIENCE PRIMERS. [the 

Lessons, you may for yourselves make out some of 
the most important changes which are in progress 
on the floor of the sea. For example, try to think 
what must become of all the wasted rock which is 
every year removed from the surface of the land. 
It is carried into the sea by streams, as you have 
now learnt. But what happens to it when it gets 
there? From the time when it was loosened from 
the sides of the mountains, hills, or valleys, this 
decomposed material has been seeking, like water, 
to reach a lower level. On reaching the hollows of 
the sea -bottom it cannot descend any further, but 
must necessarily accumulate there. 

239. It is evident, then, that between the floor of 
the sea and the surface of the land, there must be 
this great difl"erence : that. whereas the land is under- 
going a continual destruction of its surface, from 
mountain-crest to sea-shore, the sea-bottom, on the 
other hand, is constantly receiving fresh materials 
on its surface. The one is increased in proportion 
as the other is diminished. So that even without 
knowing anything regarding what men have found 
out by means of deep soundings, you could confi- 
dently assert that every year there must be vast 
quantities of gravel, sand, and mud laid down 
upon the floor of the sea, because you know that 
these materials are worn away from the land. 

240. Again, you have learnt that the restless agita- 
tion of the sea is due to movements of the air, and 
that the destruction which the sea can effect on the 
land is due chiefly to the action of the waves caused 
by wind. But this action must be merely a surface 
one. The influence of the waves cannot reach to 



SEA.] PHYSICAL GEOGRAPHY. 99 

the bottom of the deep sea. Consequently that 
bottom lies beyond the reach of the various kinds 
of destruction which so alter the face of the land. 
The materials which are derived from the waste of 
the land can lie on the sea-floor without further dis- 
turbance than they may suffer from the quiet flow of 
such ocean currents as touch the bottom. 

241. In what way, then, are the gravel, sand, and 
mud disposed of when they reach the sea ? 

242. As these materials are all brought from the 
land, they accumulate on those parts of the sea-floor 
which border the land, rather than at a distance. We 
may expect to find banks of sand and gravel in 
shallow seas and near land, but not in the middle of 
the ocean. 

243. You may form some notion, on a small scale, 
as to how the materials are arranged on the sea- 
bottom, by examining the channel of a river in a 
season of drought. At one place, where the current 
has been strong, there may be a bank of gravel ; at 
another place, where the currents of the river have 
met, you will find, perhaps, a ridge of sand which 
they have heaped up ; while in those places where 
the flow of the stream has been more gentle, the 
channel may be covered with a layer of fine silt or 
mud. You remember that a muddy river may be 
made to deposit its mud if it overflows its banks so 
far as to spread over flat land which checks its flow 
(Art. 178). 

244. The more powerful a current of water, the 
larger will be the stones it can move along. Hence 
coarse gravel is not likely to be found over the bottom 
of the sea, except near the land, where the waves can 



loo SCIENCE PRIMERS. [the 

sweep it out into the path of strong sea-currents. 
Sand will be carried further out, and laid down in great 
sheets, or in banks. The finer mud and silt may be 
borne by currents for hundreds of miles before at 
last settling down upon the sea-bottom. 

245. In this way, according to the nearness of the 
land and the strength of the ocean-currents, the sand, 
mud, and gravel worn from the land are spread out 
in vast sheets and banks over the bottom of the sea. 

246. But the sea is full of life, both of plants 
and animals. These organisms die, and their re- 
mains necessarily get mixed up with the different 
materials laid down upon the sea-floor. So that, 
besides the mere sand and mud, great numbers of 
shells, corals, and the harder parts of other sea- 
creatures must be buried there, as generation after 
generation comes and goes. 

247. It often happens that on parts of the sea-bed 
the remains of some of these animals are so abun- 
dant that they themselves form thick and wide- 
spread deposits. Oysters, for example, grow thickly 
together ; and their shells, mingled with those of 
other similar creatures, form what are called shell - 
banks. In the Pacific and the Indian Oceans a 
little animal, called the coral-polyp, secretes a hard 
limy skeleton from the sea-water ; and as millions of 
these polyps grow together, they form great reefs of 
solid rock, which are sometimes, as in the Great 
Barrier Reef of Australia, hundreds of feet thick 
and a thousand miles long. It is by means of the 
growth of these animals that those wonderful rings 
of coral-rock or Coral-islands (Fig. 18) are formed 
in the middle of the ocean. Again, a great part of 



SEA.] 



PHYSICAL GEOGRAPHY. 



lOI 



the bed of the Atlantic Ocean is covered with fine 
mud, which on examination is found to consist almost 
wholly of the remains of very minute animals called 
Foraminifera. 




Fig. i8. — Island formed by the Growth of Coral. 

248. Over the bottom of the sea, therefore, great 
beds of sand and mud, mingled with the remains 
of plants and animals, are always accumulating. If 
now this bottom could be raised up above the sea- 
level, even though the sand and mud should get as 
dry and hard as any rock among the hills, you would 
be able to say with certainty that they had once been 
under the sea, because you would find in them tlie 
shells and other remains of marine animals. 

249. You will afterwards learn when you come to 
the science of Geology that this raising of the sea- 
bottom has often taken place in ancient times. You 
will find most of the rocks of our hills and valleys to 
have been originally laid down in the sea, where they 
were formed out of sand and mud dropped on the sea- 
floor, just as sand and mud are carried out to sea and 
laid down there now. And in these rocks, not merely 
near the shore, but far inland, in quarries or ravines. 



I02 SCIENCE PRIMERS. [inside of 

or the sides and even the tops of hills, you will be 
able to pick out the skeletons and fragments of 
the various sea-creatures which were living in the 
old seas.' 

250, Since the bottom of the sea forms the great 
receptacle into which the mouldered remains of the 
surface of the land are continually carried, it is plain 
that if this state of things were to go on without 
modification or hindrance, in the end the whole of 
the solid land would be worn away, and its remains 
would be spread out on the sea-floor, leaving one 
vast ocean to roll round the globe. 

251. But there is in nature another force which 
here comes into play to retard the destruction of the 
land. We must in the remaining Lessons of this 
book consider what this force is, and how it works. 



THE INSIDE OF THE EARTH. 

252. In the foregoing pages your attention has 
been given to the surface of the earth, and what 
goes on there. Let us now consider for a little 
w^hat can be learnt regarding the inside of the earth. 

253. It may seem, at first, as if it were hopeless 
that man should ever know anything about the earth's 
interior. Just think what a huge ball this globe of 
ours is, and you will see that after all, in living and 
moving over its surface, we are merely like flies walk- 
ing over a great hill. All that can be seen from the 
top of the highest mountain to the bottom of the 



THE EARTH.] PHYSICAL GEOGRAPHY. 103 

deepest mine is not more in comparison than the 
mere varnish on the outside of a school-globe. And 
yet a good deal can be learnt as to what takes place 
within the earth. Here and there, in different coun- 
tries, there are places where communication exists 
between the interior and the surface ; and it is from 
such places that much of our information on this 
subject is derived. 

254. You have, no doubt, read of Volcanoes or 
Burning-mountains (fig. 19). These are among 
the most important of the channels of communication 
with the interior. 

255. Let us suppose that you were to visit one of 
these volcanoes just before what is called ''an eruption." 
As you approach it, you see a conical mountain, seem- 
ingly with its top cut off. From this truncated sum- 
mit a white cloud rises. But it is not quite such a 
cloud as you would see on a hill-top in this countr)\ 
For as you watch it you notice that it rises out of 
the top of the mountain, even though there are no 
clouds to be seen anywhere else. Ascending from 
the vegetation of the lower grounds, you find the 
slopes to consist partly of loose stones and ashes, 
partly of rough black sheets of rock, like the slags of 
an iron furnace. As you get nearer the top the ground 
feels hot, and puffs of steam, together with stifling 
vapours, come out of it here and there. At last you 
reach the summit, and there what seemed a level top 
is seen to be in reality a great basin, with steep 
walls descending into the depths of the mountain. 
Screening your face as well as possible from the hot 
gases which almost choke you, you creep to the top 
of this basin, and look down into it. Far below, at 

10 



i04 



SCIENCE PRIMERS. 



[inside of 



the base of the rough red and yellow cliffs which 
form its sides, lies a pool of some liquid, glowing with 
a white heat, though covered for the most part with 
a black crust like that seen on the outside of the 
mountain during the ascent. From this fiery pool jets 
of the red-hot liquid are jerked out every now and 




Fig. 19.— View of a ^^olcano. Mount Vesuvius as it -ppears at the 
present time, when viewed from the south. 



then, stones and dust are cast up into the air, and fall 
back again, and clouds of steam ascend from the same 
source and form the uprising cloud which is seen from 
a great distance hanging over the mountain. 
' 256. This caldron-shaped hollow on the summit of 
the mountain is the Crater. The intensely heated 
liquid in the sputtering boiling pool at its bottom is 
melted rock or Lava. And the fragmentary materials 
■ — ashes, dust, cinders, and stones — thrown out, are 
torn from the hardened sides and bottom of the crater 



THE EARTH.] PHYSICAL GEOGRAPHY. 105 

by the violence of the explosions with which the 
gases and steam escape. 

257. The hot air and steam, and the melted mass 
at the bottom of the crater, show that there must be 
some source of intense heat underneath. And as the 
heat has been coming out for hundreds, or even thou- 
sands of years, it must exist there in great abundance. 

258. But it is when the volcano appears in active 
eruption that the power of this underground heat 
shows itself most markedly. For a day or two before- 
hand, the ground around the mountain trembles. At 
length, in a series of violent explosions, the heart of 
the volcano is torn open, and perhaps its upper part 
is blown into the air. Huge clouds of steam roll 
away up into the air, mingled with fine dust and red- 
hot stones. The heavier stones fall back again into 
the crater or on the outer slopes of the mountain, but 
the finer ashes come out in such quantity, as sometimes 
to darken the sky for many miles round, and to settle 
down over the surrounding country as a thick cover- 
ing. Streams of white-hot molten lava run down the 
outside of the mountain, and descend even to the 
gardens and houses at the base, burning up or over- 
flowing whatever lies in their path. This state of 
matters continues for days or weeks, until the volcano 
exhausts itself, and then a time of comparative quiet 
comes when only steam, hot vapours, and gases are 
given off. 

259. About 1800 years ago, there was a mountain 
near Naples shaped like a volcano, and with a large 
crater covered with brushwood (fig. 20). No one had 
ever seen anv steam, or ashes, or lava come from it, 
and the people did not imagine it to be a volcano, like 



io6 



SCIENCE PRIMERS. 



[inside of 



some other mountains in that part of Europe. They 
had built villages and towns around its base, and 
their district, from its beauty and soft climate, used 
to attract wealthy Romans to build villas there. 
But at last, after hardly any warning, the whole of 
the higher part of the m.ountain was blown into 
the air with terrific explosions. Such showers of 
fine ashes fell for miles around, that the sky was as 
dark as midnight. Day and night the ashes and 




Fig. 20. -Vesuvius as it appeared before Pompeii was destroyed. 



Stones descended on the surrounding country ; many 
of the inhabitants were killed, either by stones falling 
on them, or from suffocation by the dust. When at 
last the eruption ceased, the district, which had 
before drawn visitors from all parts of the old world, 
was found to be a mere desert of grey dust and 
stones. Towns and villages, vineyards and gardens, 
were all buried. Of the towns, the two most noted 
were called Herculaneum and Pompeii. So com- 



THE EARTH.] PHYSICAL GEOGRAPHY. 107 

pletely did they disappear, that, although important 
places at the time, their very sites were forgotten, 
and only by accident, after the lapse of some fifteen 
hundred years, were they discovered. Excavations 
have since that time been carried on, the hardened 
volcanic accumulations have been removed from the 
old city, and you can now walk through the streets 
of Pompeii again, with their roofless dwelling-houses 
and shops, theatres and temples, and mark on the 
causeway the deep ruts worn by the carriage wheels 
of the Pompeians eighteen centuries ago. Beyond 
the walls of the now silent city rises Mount Vesuvius, 
with its smoking crater, covering one-half of the old 
mountain which was blown up when Pompeii dis- 
appeared (See fig. 19.) 

260. Volcanoes, then, mark the position of some of 
the holes or orifices, whereby heated materials from 
the inside of the earth are thrown up to the surface. 
They occur in all quarters of the globe. In Europe, 
besides Mount Vesuvius, which has been more or less 
active since it was formed, Etna, Stromboli, and 
other smaller volcanoes, occur in the basin of the 
Mediterranean, while far to the north-west some active 
volcanoes rise amid the snows and glaciers of Iceland. 
In America a chain of huge volcanoes stretches down 
the range of mountains which rises from the western 
margin of the continent. In Asia they are thickly 
grouped together in Java and some of the surround- 
ing islands ; and stretch thence through Japan and 
the Aleutian Isles, to the extremitv of North America. 
If you trace this distribution upon the map, you will 
see that the Pacific Ocean is girded all round with 
volcanoes. 



io8 SCIENCE PRIMERS. [inside of 

261. Since these openings into the interior of the 
earth are so numerous over the surface, we may 
conclude that this interior is intensely hot. But we 
have other proofs of this internal heat. In many 
countries hot springs rise to the surface. Even in 
England, which is a long way from any active 
volcano, the water of the wells of Eath is quite 
warm (120° Fahr.). It is known, too, that in all coun- 
tries the heat increases as we descend into the earth. 
The deeper a mine the warmer are the rocks and 
air at its bottom. If the heat continues to increase 
in the same proportion, the locks must be red hot 
at no great distance beneath us, 

262. It is not merely by volcanoes and hot-springs, 
however, that the internal heat of the earth affects 
the surface. The solid ground is made to tremble, 
or is rent asunder, or upheaved or let down. You 
have probably heard or read of earthquakes : 
those shakings of the ground, which, when they are at 
their worst, crack the ground open, throw down trees 
and buildings, and bury hundreds or thousands of 
people in the ruins. Earthquakes are most common 
in or near those countries where active volcanoes exist. 
They frequently take place just before a volcanic 
eruption. 

263. Some parts of the land are slowly rising out 
of the sea ; rocks, which used always to be covered . 
by the tides, come to be wholly beyond their limits ; 
while others, which used never to be seen at all, 
begin one by one to show their heads above water. 
On the other hand some tracts are slowly sinking ; 
piers, sea-walls, and other old landmarks on the 
beach, are one after apother enveloped by the sea 



THE EARTH.] PHYSICAL GEOGRAPHY. 109 

as it encroaches further and higher on the land. 
These movements, whether in an upward or down- 
ward direction, are likewise due in some way to the 
internal heat. 

264. Now when you reflect upon these various 
changes you will see that through the agency of 
this same internal heat land is preserved upon the 
face of the earth. If rain and frost, rivers, glaciers, 
and the sea were to go on wearing down the 
surface of the land continually without any counter- 
balancing kind of action, the land would neces- 
sarily in the end disappear, and indeed would have 
disappeared long ago. But owing to the pushing out 
of some parts of the earth's surface by the move- 
ments of the heated materials inside, portions of the 
land are raised to a higher level, while parts of tlie 
bed of the sea are actually upheaved so as to form 
land. 

265. Tliis kind of elevation has happened many 
times in all quarters of the globe. As already men- 
tioned (Art. 249), most of our hills and valleys are 
formed of rocks, which were originally laid down on 
the bottom of the sea, and have been subsequently 
raised into land. 

CONCLUSION. 

266. In conclusion, let us sum up the leading 
features of the foregoing Lessons. 

267. This earth of ours is the scene of continual 
movement and change. The atmos])here which 
encircles it is continually in motion, diffusing heat, 
light, and vapour. From the sea and from the waters 



no SCIENCE PRIMERS. [conclusion. 

of the land, vapour is constantly passing into the air, 
whence, condensed into clouds, rain and snow, it 
descends again to the earth. All over the surface of 
the land the water which falls from the sky courses 
seawards in brooks and rivers, bearing into the great 
deep the materials which are worn away from the land. 
Water is thus ceaselessly circulating between the air, 
the land, and the sea. The sea, too, is never at 
rest. Its waves gnaw the edges of the land, and its 
currents sweep round the globe. Into its depths the 
spoils of the land are borne, there to gather into rocks, 
out of which new islands and continents will even- 
tually be formed. Lastly, inside the earth is lodged a 
vast store of heat by which the surface is shaken, 
rent open, upraised or depressed. Thus while old 
land is submerged beneath the sea, new tracts are up- 
heaved, to be clothed with vegetation and peopled 
with animals, and to form a fitting . abode for man 
himself. 

268. This world is not a living being, like a plant 
or an animal, and yet you must now see that there 
is a sense in which we may speak of it as such. The 
circulation of air and water, the interchange of sea 
and land \ in short, the system of endless and con- 
tinual movement by which the face of the globe is 
day by day altered and renewed, may well be called 
the Life of the Earth. 



QUESTIONS. 

THE SHAPE OF THE EARTH, p. 8. 

1. What is the first impression we have of the shape of the 
Earth ? 

2. How could you show in the interior of a level country that 
the apparent plain is really part of the surface of a globe? 

3. Prove the same conclusion from what may be seen on the 
sea-coast. 

4. How has the shape of the earth been tested by "circum- 
navigators ? ' ' 

5. Show how the gentle curvature indicates the size of the 
globe. 

6. How long would a railway train moving at a rate of thirty 
miles an hour take to go round the earth ? 

DAY AND NIGHT, p. 13. 

1. Whence does the earth derive its surface-heat and light ? 

2. What was the ancient belief as to the relative positions of 
the earth, sun, moon, and stars? 

3. Are there any traces of this early belief ctill to be found in 
our everyday speech ? 

4. What is the real relation of tbe sun to the earth ? 

5. The succession of day and night appears as if it were due 
•to the movement of the sun across the sky ; illustrate how it is 

really caused by the motion of the earth. 

6. What is meant by the terms axis of rotation, north pole 
and south pole ? 

7. In what direction is the earth rotating ? How is this indi- 
cated by day and at night respectively? 

8. What is the earth's motion of reziolution ? 

9. In what time does the earth perform a complete revolu- 
tion ? 

10. Show how the movements of the earth determine our 
divisions of time. 



112 SCIENCE PRIMERS. [questions. 



THE AIR. 

I. What the Air is made of, p. i6. 

1. What is meant by the term Atmosphere ? 

2. Of what materials is the air mainly composed ? 

3. Besides the two chief gases, name some other substances 
alA^ays present in the air, 

4. How may the presence of visible particles be shown ? 

5. What is water-vapour? [See art. 73.] Show by any 
familiar example how it may be invisibly dissolved in the air. 
[See art, 71.] 

6. In what proportion does carbonic acid gas occur in the 
air? 

7. Show how important this material is in relation to the 
growth of plants and animals. 

II. The Warming and Cooling of the Air, p, ig. 

1. In what ways are we made sensible of the presence of the 
air? 

2. Why do we feel cold when we pass from a warm room into 
the outer air in winter? 

3. The sun is always radiating heat to the earth ; why then 
should there be alternations of heat and cold in the air ? 

4. Does the atmosphere allow the whole of the sun's heat-rays 
to pass through it to the surface of the earth ? 

5. Why is the sun's heat less felt in the morning and in the 
evening than at noon ? 

6. Why is night so much colder than day ? 

7. Why is summer warmer than winter ? 

8. Why is it that cloudy days are not always or necessarily 
cold ? 

9. Since the air absorbs only part of the heat of the sun which 
passes through it to the earth's surface, how is it chiefly warmed 
and how cooled? [See art. 64.] 

ID. What prevents excessive loss of heat at night by radia- 
tion ? 

11. Why are the nights often felt to be so cold in warm 
countries ? 

12. Why are cloudy nights usually warmer than clear ones? 

III. What happens when Air is warmed or cooled. 
Wind, p. 24. 

I. Whether is warm or cold air the heavier, and why? 



QUESTIONS.] PHYSICAL GEOGRAPHY. 113 

2. What is the general effect of difference of density in causing 
movement of the air ? 

3. How do a red-hot poker and a common fire-place illus- 
trate this movement ? 

4. How does wind arise from the unequal heating of the 
earth's surface ? 

5. Explain the nature and origin of land and sea breezes. 

6. Which is the hottest belt of the earth's surface, and why ? 

7. Explain the nature and origin of the trade winds. 

8. How does water-vapour cause movements in the atmo- 
sphere ? 

* 

IV. The Vapour in the Air. Evaporation and Con- 
densation, p. 27. 

1. Explain why a film of mist appears on a cold glass when 
brought into a warm room, 

2. How does the capacity of the air to retain water-vapour 
vary according to temperature ^ 

3. Why does a film of mist appear upon a mirror or other 
cold surface when it is breathed on, and what is the explanation 
of the cloud which issues from one's mouth with every breath in 
cold weather. 

4. What is the dew-point ? 

5. How is the vapour of water brought into the air? 

6. At what times is evaporation most and least vigorous ? 

7. Explain the cause of the chill that is felt when a drop of 
water is evaporated on the back of the hand ? 

V. Dew, Mist, Clouds, p. 31. 

1. Give some exam.ples of the condensation of vapour. 

2. Explain the formation of dew. 

3. Show how mists are formed upon mountains 

4. Explain the origin of the fog often seen rising after sunset 
from the surface of a river. 

5. Explain the formation of clouds. 

VI. Where Rain and Snow come from, p. 35. 

1. In what ways do clouds disappear from the sky ? 

2. "Explain the formation and fall of rain. 

3. Under what different forms does water present itself? 

4. What is ice and when is it formed ? 

5. What is snow ? Describe a snow-flake. 



114 SCIENCE PRIMERS. [questions. 

6. What are hail and sleet ? 

7. Describe the circulation of water between the air and the 
earth. 

THE CIRCULATION OF WATER ON THE LAND. 
I. What becomes of the Rain, p. 39 

1. Why do not seas, lakes, and rivers, become visibly less, 
seeing that they lose so much water by evaporation ? 

2. What part does the sea play in supplying the air with 
moisture ? 

3. What becomes of that part of the rain which falls into the 
sea? 

4. How much rain is estimated to fall annually upon the 
British Isles? 

5. How is the rain which falls upon land disposed of? [See 
art. 106.] 

6. How may it be shown that a considerable quantity of rain 
sinks into the ground, and yet that this quantity is not perma- 
nently removed from the circulation ? 

II. How Springs are formed, p. 42. 

1. How do sand and clay differ from each other in regard to 
the passage of water through them? 

2. How does this difference affect the kinds of soil? 

3. What inference as to the movements of the underground 
water, may be drawn from the fact that water gathers in any 
deep hole or quarry which may be dug out of the ground? 

4. What natural channels are provided for the passage of 
water, even through very hard rocks ? 

5. Explain the occurrence of boggy places in hilly ground. 

6. What are springs? 

7. Explain why springs issue from between beds of rock along 
the sides of valleys. 

8. Explain the origin of deep-seated springs. 

9. How is the underground circulation of water shown by 
wells, mines, and pits ? 

III. The work of Water Underground, p. 47. 

1. Does clear spring- water contain anything else than water ? 
How may this be answered practically ? 

2. What common solutions show that clear transparent water 
may contain a good deal of foreign matter invisible to the eye ? 



QUESTIONS.] PHYSICAL GEOGRAPHY. iij 



3. Whence must the substances dissolved in spring-water be 
derived ? 

4. What part does rain play in regard to the purification of 
the air ? 

5. Whence does rain-water derive the carbonic acid which it 
carries below the soil ? 

6. What effect has water containing carbonic acid on many 
rocks ? 

7. Explain this action of water in limestone countries. 

8. What is the difference between hard and soft water? 

9. Are the substances carried up from below in spring- water 
of any service in the growth of plants and animals ? 

10. What is the origin of underground tunnels and caverns ? 

IV. How the Surface of the Earth crumbles away, 

p. 51. 

1. What change usually takes place upon masonry after it has 
been exposed for a time to the air? 

2. Show how a similar change can be obsers'ed elsewhere than 
in human erections. 

3. Explain the part taken by carbonic acid in the crumbling 
of the rocks at the surface of the earth. 

4. Explain the eftect of the oxygen in rain-water upon iron 
and on many rocks. 

5. Explain the action of frost in promoting the crumbling of 
soil and the splitting up of rocks. 

6. What is the effect of rapid extremes of heat and cold upon 
rocks ? 

7. State the general result of all these destructive agents 
upon the surface of the land, and show how their action is bene- 
ficial in making the earth a fit dwelling-place for plants and 
animals. 

V. What becomes of the Crumbled Parts of Rocks. 
How Soil is made, p. 58. 

1. What is common garden soil made of? 

2. What is meant by the chemical action of rain ? 

3. Explain the mechanical action of rain. 

4. What is the nature of the process by which soil is made? 

5. Explain how soil is continually renewed. 

6. Show how plants lend their help in the making of soil. 

7. What part do common earth-worms play in the process? 

8. In what sense may it be said that the general surface of 
the land is continually moving towards the sea ? 

9. How do brooks and rivers illustrate the extent to which the 
surface of the land is mouldering ? 

11 



ii6 SCIENCE PRIMERS. [questions. 

VI. Brooks and Rivers. Their Origin, p. 62. 

1. Describe the formation of miniature brooks and rivers on 
a sloping roadway during a heavy shower of rain. 

2. Why do streams flow? 

3. What are lakes ? 

4. Why does the rain run off the surface of the land in 
runnels, brooks, and rivers ? 

5. How are the innumerable brooks of the high ground 
disposed of as they descend towards the lo ^ver country ? 

6. What is meant by a water-shed ? 

7. Why do rivers continue to flow even in dry weather ? 

8. Why are some rivers, such as the Rhine, most swollen in 
summer? 

9. What becomes of all the surplus drainage of the land ? 

VII. Brooks and Rivers. Their Work, p. 68. 

1. Give an illustration of the vast amount of invisible material 
carried, in chemical solution, by a river to the sea. 

2. Why are rivers discoloured during floods ? 

3. What is the origin of the gravel and blocks of stone in 
the bed of a stream, and why are the stones usually rounded ? 

4. What are pot-holes ? 

5. How have river gorges and ravines been formed ? 

6. Describe the bed of a river when the water is low. 

7. Explain the origin of the flat terraces bordering a river. 

8. Describe a delta, and show how it may be formed at the 
mouth of a river, in a lake, or in the sea. 

9. What becomes of the mud and sand which are carried past 
the delta? 

VIII. Snowfields and Glaciers, p. 75. 

1. What is meant by the snow-line ? 

2. What is its height at the equator and in the polar regions ? 

3. Why does snow remain perpetual above the snow-line ? 

4. In what way does the snow below the snow-line disappear ? 

5. How may the sudden melting of snow prove very destruc- 
tive? 

6. What becomes of the mass of snow which accumulates 
above the snow-line ? 

7. Describe the formation of a glacier. 

8. What becomes of a glacier as it descends its valley ? 

9. What are moraines ? 

10. How do stones and earth get under the ice of a glacier? 

11. What use does the glacier make of these stones and 
particles of earth and sand ? 



Q-JESiiuNS.] PHYSICAL GEOGRAPHY. 117 

12. Why is the river of water muddy which escapes from the 
end of a glacier ? 

13. Where do the largest glaciers exist ? 

14. Explain the formation of icebergs. 

15. What proofs have been found that glaciers once existed 
in countries such as Britain, where they no longer occur? 

THE SEA. 

I. Grouping of Sea and Land, p. 86. 

1. What are the proportions of land and water on the earths 
surface ? 

2. Mention the broad difference between sea and land in the 
way they are distributed over the globe. 

3. On which side of the equator does most of the land lie ? 

4. What part of the earth's surface lies in the centre of the 
land hemisphere ? 

5. What are continents and islands? 

6. What are oceans ? 

II. Why the Sea is Salt, p. 88. 

1. In what familiar respect does the water of the sea differ 
from that of ordinary springs and rivers ? 

2. What happens when a drop of sea-water is evaporated on 
a piece of glass ? 

3. Whence h -s the miner<?l matter in sea- water come ? 

4. What is the relative salness of the Atlantic Ocean and t^e 
Dead Sea? 

III. The Motions of the Sea, p. 90. 

1. What is the commonest and most obvious form of motion 
in the sea ? 

2. How does the ebb and flow of the tides show itself on a 
sloping beach ? 

3. What is surface drift, and how is it often indicated ? 

4. What are currents in the sea, and how are they sometimes 
made evident ? 

5. How may a basin or trough of water be made to illustrate 
the formation of waves ? 

6. What is the connection between movements of the air and 
ripples or waves on the sea ? 

7. What general effect have waves on the edge of the land 
exposed to their influence? 



ii8 SCIENCE PRIMERS. [QUEbTicss. 

8. Explain the process by which gravel and sand are ground 
down by the waves upon the beach. 

9. How do the waves wear down a rocky coast ? 

IV. The Bottom of the Sea, p. 95. 

1. What is the general character of the sea-floor as compared 
with the surface of the land ? 

2. How is our information regarding the bottom of the deep 
sea obtained ? 

3. What was found to be the depth of the Atlantic Ocean 
Avhen soundings were made for the telegraphic cable between 
Britain and America ? 

4. What is the greatest depth that has yet been observed in 
the Atlantic, and where does it occur ? 

5. What is the depth of a great part of the sea? 

6. Which are usually the deepest and which the shallowest 
parts of the sea ? 

7 What is the depth of the deeper parts of the North Sea? 

8. How much of St. PauFs Cathedral in London would be 
submerged were it placed in the middle of the -Straits of 
Dover ? 

9. What is a di-edge, and what use is made of it ? 

10. What light has been obtained by means of the dredge 
regarding the living things of the deep sea bottom ? 

n. Mention an important difference between the crumbling 
land-surface described in a former lesson [arts. 123 — 142], and 
the bottom of the sea. 

12. To what part of the sea is the destructive action of the 
waves limited ? 

13. How are the mud, earth, sand, and gravel disposed of 
M^hich the sea obtains from the crumblinfj surface of the land? 

14. What becomes of the remains of the shells, corals, and 
other creatures on the sea-floor ? 

15. What are shell-banks ? , 

16. What are coral-reefs and coral-islands, and how are they 
formed ? 

17. What is the nature of the mud which covers a great part 
of the bed of the Atlantic? 

18. How could you be certain that some rocks must once have 
been under the sea ? 

THE INSIDE OF THE EARTH, p. 102 

I. Does the distance from the top of the highest mountain 
to the bottom of the deepest mine bear a large proportion to 
the diameter of the whole globe ? 



QUESTIONS.] PHYSICAL GEOGRAPHY. 119 



2. What is a volcano ? 

3. What various materials are 'throNvn out by a volcano ? 

4. What evidence do these materials furnish as to the condition 
of the earth's interior? 

5. Describe a volcanic eruption. 

6. What has been the history of Vesuvius? 

7. State the position of some of the volcanoes of Europe, 
America, and Asia. 

8. What evidence do hot springs bring to bear upon the 
state of the iniernal parts of our globe ? 

9. What has been observed regarding temperature as we 
descend into the earth, and what inference has been drawn 
from it ? 

10. What are earthquakes? Where are they most frequent ? 

11. Mention any facts which show that different pans of the 
earth's surface are slowly changing their level. 

12. In what way does the action of the earth's internal heat 
tend to counteract the general lowering of level caused by the 
destructive action of air, rain, frosts, rivers, glaciers, and the 
sea? 

13. Under what circumstances were the rocks of most of our 
hills and valleys formed? 



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PHYSICS. Balfoiu* Stewart. 
PHYSICAI. GEOGRAPHY. 
GEOLOGY. A. Geikie. 
PHYSI0I*06Y. M. Fos 
ASTRONOMY. J. N. Loc 
BOTANY. J. D. Hooker. 
liOGIC. W. S. Jevon .. 
INVEWTI.ONAX GEOME 
PIANOFORTE. Franklin Ta 
P0IJ:TICAI* economy. W. &. Jevons. 
NATURAL RESOURCES OF THE UNITED 
J. H. Patton. 



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GREECE. C. A. FyfFe. 

ROME. M. Creighton. 

EUROPE. £. A. Freeman. 

OLD GREEK LIFE. J. P. Mahaliy. 

ROMAN ANTIQUITIES. A. S. Wilkina. 

GEOGRAPHY. George Grove. 

Literature Primer,. 

ENGLISH GRAMMAR. R. Morris. 
ENGLISH LITERAl*UR£. Stopford A. Bi 
PHILOLOGY. J. PeUe. 
CLASSICAL GEOGRAPHY. M. F. Toxcr, 
SHAB;ESP£AR£. £. Dowden. 
STUDIES IN BRYANT. J. Alden. 
GREEK LITERATURE. R. C Jebb. 

NGLISH GRAMMAR EXERCISES. R. MorriiP. 
UOWSR* W. E. Gladstone. 
ENGLISH COMPOSITION. J. Nichol. 

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